More Important Topics of Cylinder

The skeleton changes composition over a lifespan. Early in gestation, a fetus has no hard skeleton - bones form gradually during nine months in the womb. When a baby is born it has more bones than it will as an adult On average, an adult human has 206 bones in their skeleton (the number can vary slightly from individual to individual), but a baby is born with approximately 270. The difference comes from a number of small bones that fuse together during growth. These include the bones in the skull and the spine.

The sacrum (the bone at the base of the spine) consists of six bones which are separated at birth but fuse together into a solid structure in later years.

There are 6 bones (three on each side) in the middle ear that articulate only with themselves, and one bone, the hyoid bone, which does not touch any other bones in the body.

The longest bone in the body is the femur and the smallest is the stapes bone in the middle ear.

Function

The skeleton functions not only as the support for the body but also in haematopoiesis, the manufacture of blood cells that takes place in bone marrow. This is why people who have cancer of the bone marrow almost always die. It is also necessary for protection of vital organs and is needed by the muscles for movement.

Gender differences

There are many differences between the male and female human skeletons, some more noticeable than others. Men tend to have slightly thicker and longer limb and digit bones while women tend to have larger pelvic bones in relation to body size. Women also tend to have narrower rib cages, smaller teeth, less angular mandibles, and less pronounced cranial features such as the brow ridges and occipital condyle (the small bump in the cranium's posterior). There are also a number of smaller differences between human male and female skeletons.

There is a myth that men have one less rib than women. This stems from a passage found in the Bible which states that Eve was created from one of Adam's ribs. However, both men and women have the same number of ribs: 12 pairs or 24 total.

Organization

One way to group the bones of the human skeleton is to divide them into two groups, namely the axial skeleton and the appendicular skeleton. The axial skeleton consists of bones in the midline and includes all the bones of the head and neck, the vertebrae, ribs and sternum. The appendicular skeleton consists of the clavicles, scapulae, bones of the upper limb, bones of the pelvis and bones of the lower limb.

The bones of the human skeleton are structurally and in many taxonomies organized as those of the: skull, middle ear, throat, shoulder girdle, vertebra, arms, hands, pelvis , legs, feet

Diseases

The skeleton can be affected by many diseases that compromise physical mobility and strength. Skeletal diseases range from minor to extremely debilitating. Bone cancer and bone tumors are extremely serious and are sometimes treated by radical surgery such as amputation of the affected limb.

Various forms of arthritis attack the skeleton resulting in severe pain and weakness. Osteoporosis can increase the likelihood of fractures and broken bones, especially among post-menopausal women and the elderly

Forms your forehead and the roof of your eye sockets

Flexible skull: Skull bones aren't fused together at birth

Mobile mandible: Your mandible, or jawbone, is the only bone in your skull that moves

Two sets of bones

Your skull is made up of two sets of bones - the bones of your face and the bones of your cranium, which make up your forehead and the back of your head.

Cranium

Your cranium is the large bony case that surrounds your delicate brain, protecting it from bumps and knocks. It is made up of eight large flat bones, joined together by fixed joints known as sutures. Your frontal bone forms your

forehead, and the tops of your eye sockets. Most of the top and sides of your head are formed by two parietal bones. And the back of your skull is formed by your occipital bone which has an opening in it where your spinal cord connects to your brain.

Facial bones

The fourteen bones at the front of your skull hold your eyes in place and form your facial features. Your mandible, or jawbone, is the largest, strongest bone in your face. It holds your lower teeth in place and you move it to chew your food.

Apart from you mandible and your vomer, all your facial bones are arranged in pairs. That's why your face is symmetrical.

For example, your two zygomatic bones form your cheekbones and the outside of your eye sockets on either side of your face.

From flexible to fixed joints

A human skull is almost full sized at birth. However the eight bones that make up the cranium are not yet fused together. This means that the skull can flex and deform during birth, making it easier to deliver a baby through the narrow birth canal. These individual plates of bone fuse together after about 24 months to form the adult skull.

The only bone in your skull that forms freely movable joints is your mandible, or jawbone.

The jaw is either of the two opposable structures forming, or near the entrance to, the mouth. In most vertebrates, the jaws are bony or cartilaginous and oppose vertically, comprising an upper jaw and a lower jaw. In arthropods, the jaws are chitinous and oppose laterally, and may consist in mandibles,

chelicerae, or, loosely, pedipalps. Their function is fundamentally for food acquisition, conveyance to the mouth, and/or initial processing (mastication or chewing). The term jaws is also broadly applied to the whole of the structures constituting the vault of the mouth and serving to open and close it.

In vertebrates, the lower jaw or mandible is the mobile component that articulates at its posterior processes, or rami (singular ramus), with the temporal bones of the skull on either side; the word jaw used in the singular typically refers to the lower jaw.

The upper jaw or maxilla is more or less fixed with the skull and is composed of two bones, the maxillae, fused intimately at the median line by a suture; incomplete closure of this suture and surrounding structures may be involved in the malformation known as cleft palate. The maxillary bones form parts of the roof of the mouth, the floor and sides of the nasal cavity, and the floor of the orbit or eye socket. The jaws typically accommodate the teeth or form the bases for the attachment of a beak.

Viewed laterally the vertebral column presents several curves, which correspond to the different regions of the column, and are called cervical, thoracic, lumbar, and pelvic. The cervical curve, convex forward, begins at the apex of the odontoid (tooth-like) process, and ends at the middle of the second thoracic vertebra; it is the least marked of all the curves. The thoracic curve, concave forward, begins at the middle of the second and ends at the middle of the twelfth thoracic vertebra.

Its most prominent point behind corresponds to the spinous process of the seventh thoracic vertebra. The lumbar curve is more marked in the female than in the male; it begins at the middle of the last thoracic vertebra, and ends at the sacrovertebral angle. It is convex anteriorly, the convexity of the lower three vertebrae being much greater than that of the upper two.

The pelvic curve begins at the sacrovertebral articulation, and ends at the point of the coccyx; its concavity is directed downward and forward. The thoracic and pelvic curves are termed primary curves, because they alone are present during fetal life. In the early embryo, the vertebral column is C-shaped, and the cervical and lumbar curvatures are not yet

present in a newborn infant. The cervical and lumbar curves are compensatory or secondary, and are developed after birth, the former when the child is able to hold up its head (at three or four months), and to sit upright (at nine months), the latter at twelve or eighteen months, when the child begins to walk.

The vertebral column also has a slight lateral curvature, the convexity of which is directed toward the right side. This may be produced by muscular action, most persons using the right arm in preference to the left, especially in making long-continued efforts, when the body is curved to the right side.

In support of this explanation it has been found that in one or two individuals who were left-handed, the convexity was to the left side. This curvature is regarded by others as being produced by the aortic arch and upper part of the descending thoracic aorta – a view which is supported by the fact that in cases where the viscera are transposed and the aorta is on the right side, the convexity of the curve is directed to the left side.

Surfaces

Anterior surface

When viewed from in front, the width of the bodies of the vertebrae is seen to increase from the second cervical to the first thoracic; there is then a slight diminution in the next three vertebrae; below this there is again a gradual and progressive increase in width as low as the sacrovertebral angle. From this point there is a rapid diminution, to the apex of the coccyx.

Posterior surface

The posterior surface of the vertebral column presents in the median line the spinous processes. In the cervical region (with the exception of the second and seventh vertebrae) these are short and horizontal, with bifid extremities. In the upper part of the thoracic region they are directed obliquely downward; in the middle they are almost vertical, and in the lower part they are nearly horizontal.

In the lumbar region they are nearly horizontal. The spinous processes are separated by considerable intervals in the lumbar region, by narrower intervals in the neck, and are closely approximated in the middle of the thoracic region. Occasionally one of these processes deviates a little from the median line — a fact to be remembered in practice, as irregularities of this sort are attendant also on fractures or displacements of the vertebral column.

On either side of the spinous processes is the vertebral groove formed by the laminae in the cervical and lumbar regions, where it is shallow, and by the laminae and transverse processes in the thoracic region, where it is deep and broad; these grooves lodge the deep muscles of the back. Lateral to the vertebral grooves are the articular processes, and still more laterally the transverse processes.

In the thoracic region, the transverse processes stand backward, on a plane considerably behind that of the same processes in the cervical and lumbar regions. In the cervical region, the transverse processes are placed in front of the articular processes, lateral to the pedicles and between the intervertebral foramina. In the thoracic region they are posterior to the pedicles,

intervertebral foramina, and articular processes. In the lumbar region they are in front of the articular processes, but behind the intervertebral foramina.

Lateral surfaces

The lateral surfaces are separated from the posterior surface by the articular processes in the cervical and lumbar regions, and by the transverse processes in the thoracic region. They present, in front, the sides of the bodies of the vertebrae, marked in the thoracic region by the facets for articulation with the heads of the ribs.

More posteriorly are the intervertebral foramina, formed by the juxtaposition of the vertebral notches, oval in shape, smallest in the cervical and upper part of the thoracic regions, and gradually increasing in size to the last lumbar. They transmit the spinal nerves and are situated between the transverse processes in the cervical region, and in front of them in the thoracic and lumbar regions.

Vertebral canal

The vertebral canal follows the different curves of the column; it is large and triangular in those parts of the column which enjoy the greatest freedom of movement, such as the cervical and lumbar regions; and is small and rounded in the thoracic region, where motion is more limited.

Abnormalities

Occasionally the coalescence of the laminae is not completed, and consequently a cleft is left in the arches of the vertebrae, through which a protrusion of the spinal membranes (dura mater and arachnoid), and generally of the spinal cord (medulla spinalis) itself, takes place, constituting the malformation known as spina bifida. This condition is most common in the lumbosacral region, but it may occur in the thoracic or cervical region, or the arches throughout the whole length of the canal may remain incomplete.

The following abnormal curvatures may occur in some people:

Kyphosis is an exaggerated posterior curvature in the thoracic region. This produces the so-called "humpback".

Lordosis is an exaggerated anterior curvature of the lumbar region, "swayback". Temporary lordosis is common among pregnant women.

Scoliosis, lateral curvature, is the most common abnormal curvature, occurring in 0.5% of the population. It is more common among females and may result from unequal growth of the two sides of one or more vertebrae.

In anatomy, the ribs (Latin costae) are the long curved bones which form the rib cage. They surround the chest (Latin thorax) of land vertebrates. They protect the lungs, heart, and other internal organs of the thoracic cavity. In mammals, obvious ribs only occur in the chest: fused-on remnants of ribs can be traced in development in neck vertebrae and sacral vertebrae.

In reptiles, ribs sometimes occur in all vertebrae from the neck to the sacrum. In fish, the full set is four ribs on each vertebra. This can easily be seen in the herring. Not all fish have the full set. The human skeleton has 24 ribs, 12 on each side. (A small proportion have one pair more or fewer).

They are attached to the vertebral column behind. The first seven pairs are connected to the sternum in front and are known as true ribs (costae verae, I-VII). The eighth, ninth, and tenth are attached in front to the cartilaginous portion of the next rib above and are known as false ribs (costae spuriae, VIII-X). The lower two, that is the eleventh and twelfth, are not attached in front and are called floating ribs (costae fluitantes, XI-XII).

The spaces between the ribs are known as intercostal spaces; they contain the intercostal muscles, nerves, and arteries. The rib cage allows for breathing due to its elasticity. In some humans, the rib remnant of the 7th neck vertebra on one or both sides is replaced by a free extra rib called a cervical rib, which can cause trouble for the nerves going to the arm.

There is a legend that men have one rib fewer than women. This is false, and originates from the Bible's description of the creation of Eve (from the rib of Adam).

bottom (the lower half of the "loops") is the ischium. These three bones fuse together with age and are collectively known as the hip bone, ossa coxae or the innominate bone. The pelvis is joined to the sacrum bone by ligaments, and the hip bones nest in specially shaped sockets (the acetabulum) on each side.

The place at the front of the pelvis where the two sides join together is called the symphysis pubis. This is normally a very inflexible joint, but it softens and becomes more flexible during late pregnancy, allowing it to expand during labour for the baby's head to pass through. A female pelvis is also wider and shallower than a male pelvis.

One easy way to distinguish between genders is to examine the very front of the pelvis, and compare the angle formed by the bones that come from below with your fingers. If the angle is about the same as between your outstretched thumb and index finger, it is a female pelvis (arcus pubis). If it is closer to the angle between your spread index and middle fingers, it is a male pelvis (arcus subpubis).

The pelvis protects the digestive and reproductive organs in the lower part of the body, and many large nerves and blood vessels pass through it to supply the legs.

Although many mammals and other animals have grasping appendages similar in form to a hand, these are scientifically not considered to be so, and have other varying names, including paws. Using the term hand is merely a scientific usage of anthropomorphization, to distinguish the terminations of the front paws from the hind ones. The only true hands appear in the mammalian order of primates. Hands must also feature opposable thumbs, as described later in the text.

Structure of the hand

The hand consists of a broad palm (metacarpus) with five digits, attached to the forearm by a joint called the wrist (carpus).

Digits

The four fingers

The four fingers on the hand are located at the outermost edge of the palm. These four digits can be folded over the palm, this allows for the holding of

objects, and furthermore the grasping of small objects. Each finger, starting with the one closest to the thumb, has a colloquial name to distinguish it from the others:

index finger, pointer finger, or forefinger

middle finger

ring finger

little finger or pinky

The thumb

The thumb (connected to the trapezium) is located on one of the sides, parallel to the arm. The thumb can be easily rotated 90º, on a perpendicular level compared to the palm, unlike the fingers which can only be rotated approximately 45º.

A reliable way of identifying true hands is from the presence of opposable thumbs. Opposable thumbs are identified by the ability to be brought opposite to the fingers.

The phalanx bones in a human hand are visible in this x-ray

Bones. The human hand has at least 27 bones: the carpus or wrist account for 8; the metacarpus or palm contains 5; the remaining 14 are digital bones.

Bones of the wrist

The wrist has eight bones, arranged in two rows of four. These bones fit into a shallow socket formed by the bones of the forearm.

Bones of the palm

The palm has 5 bones, one to each of the 5 digits.

Digital bones

Also called phalanx bones. Human hands contain 14 of them; 2 in the thumb, and 3 in each of the four fingers, called; distal phalanx, carrying the nail, middle phalanx and proximal phalanx. (The thumb has no middle phalanx).

Sesamoid bones

Sesamoid bones are small ossified nodes embedded in the tendons to provide extra leverage and reduce pressure on the underlying tissue.

Many exist around the palm at the bases of the digits, but the exact number varies between different people. The patella is the largest example of a sesamoid bone in the human body.

Articulation

The fingers of a hand bend to hold up a tomato.Also of note is that the articulation of the human hand is more complex and delicate than that of comparable organs in any other animals. Without this extra articulation, we would not be able to operate a wide variety of tools and devices. The hand can also form a fist, for example in combat, or as a gesture.

The name Phalanges is commonly given to the bones that form fingers and toes. In primates such as humans and monkeys, the thumb and big toe have two phalanges, while the other fingers and toes consist of three.

The phalanges do not really have individual names but are named after the digit, and their distance from the body. Distal phalanges are at the tips of the fingers and toes, the proximal phalanges are closest to the hand (or foot) and articulate with the metacarpals or metatarsals. Middle phalanges are between the distal and proximal. The thumb and big toe do not have middle phalanges.

The phalanges of the foot correspond with those of the hand. They differ from them in their size (the bodies being much reduced in length) and being laterally compressed.

First Row, The body of each is compressed from side to side, convex above, concave below. The base is concave; and the head presents a trochlear surface for articulation with the second phalanx.

Second Row, The phalanges of the second row are remarkably small and short, but rather broader than those of the first row.

The ungual phalanges, in form, resemble those of the fingers; but they are smaller and are flattened from above downward; each presents a broad base for articulation with the corresponding bone of the second row, and an expanded distal extremity for the support of the nail and end of the toe.

The phalanges are each ossified from two centers: one for the body, and one for the base.

The center for the body appears about the tenth week, that for the base between the fourth and tenth years; it joins the body about the eighteenth year.

The shoulder complex comprises five individual joints. The term "shoulder" often is used specifically to refer to the principle one of these, the glenohumeral joint, consisting of the proximal ball of the humerus and the associated ("glenoid") socket of the scapula. A

bove this lies the acromioclavicular, or "a-c" joint, between the clavicle and the acromial process of the scapula. The clavicle in turn articulates with the chest wall at the sternoclavicular joint, while the scapula slides across the posterior chest wall via the "scapulothoracic" joints.

The musculature of the shoulder region is similarly complex. The muscles which drive the arm include the pectoralis major (chest muscle), which pulls the arm forward; the trapezius, which elevates the shoulder; the deltoid, which elevates the arm to the side; and the latissimus dorsi, which pulls the arm down.

The clavicular part of the pectoralis major runs downward and outward from the inner half of the clavicle. The clavicular part of the deltoid attaches to the outer third of the clavicle. Between these two muscles is an elongated triangular gap with its base at the clavicle. Here, where the skin is somewhat depressed, the cephalic vein sinks between the two muscles to join the axillary vein. The tip of the coracoid process is situated just under cover of the inner edge of the deltoid, one inch below the junction between the outer and middle thirds of the

clavicle. The deltoid muscle forms the prominence of the shoulder, and its convex outline is due to the presence of the head of the humerus deep within it. When the humerus is dislocated, the deltoid muscle's appearance becomes flattened.

The pectoralis major forms the anterior fold of the axilla or armpit, the posterior fold being formed by the latissimus dorsi and teres major muscles. The skin of the floor of this space is covered with hair in the adult, and contains many large sweat glands. The axillary vessels and brachial plexus of nerves traverse this region. Below the lateral edge of the pectoralis major, the swelling of the biceps begins to be visible. The course of this large muscle, which flexes the forearm, can easily be traced into its tendon of insertion, in the front of the elbow joint.

Arm

On the sides of the biceps lie the external and internal bicipital furrows, in the latter of which the brachial artery may be felt and compressed. The median nerve lies here, in close relation to the artery. At the bend of the elbow the two condyles of the humerus may be felt, on either side. The inner one projects beneath the skin, but the outer one is obscured by the rounded outline of the brachioradialis muscle. On the back of the arm lie the three heads of the triceps muscle, the external forming a marked oblique swelling when the forearm is forcibly extended and internally rotated. The triceps serves to "extend", or straighten, the elbow joint.

Elbow

The elbow is composed of three joints, which allow the bones of the elbow to articulate with one another. These bones are the humerus (upper arm), the ulna (medial forearm) and the radius (lateral forearm). The major articulation of the elbow is the ulno-humeral joint, which enables arm flexion and extension. Interestingly, a person can perform most daily tasks with a range of flexion from 30 degrees to 70 degrees.

Secondly, the head of the radius is dish-shaped and interacts with the capitellum (a spherical structure on the distal humerus) to form the radio-capitellar joint. This joint is responsible for adding stability to the elbow by transferring forces across the elbow joint. Thirdly, the radial head also spins against the ulna to form the proximal radio-ulnar joint, which is responsible for forearm rotation. The ulnohumeral, radiocapitellar and proximal radioulnar joints work together to position the hand in space.

On the back of the elbow, the olecranon process of the ulna is quite subcutaneous. During extension of the elbow it is in line with the two condyles. Between it and the inner condyle lies the ulnar nerve, here known popularly as the "funny bone". Striking the elbow in this area may send a shock wave of pain down the forearm.

The superficial veins at the bend of the elbow are very conspicuous. Their typical arrangement is an 'M', of which the radial and ulnar veins form the uprights, while the outer oblique bar is the median cephalic and the inner oblique the median basilic vein. At the divergence of these two the median vein comes up from the front of the forearm, while the two vertical limbs continue up the arm as the cephalic and basilic veins, on the outer and inner sides, respectively.

In the upper part of the front of the forearm lies the antecubital fossa or triangle. Its outer boundary is the brachio-radialis, its inner the pronator radii teres, and its apex the point below at which they merge. In this space are three vertical structures: externally the tendon of the biceps, just internal to this the brachial artery, and still more internally the median nerve.

Coming from the inner side of the biceps tendon the semi-lunar fascia may be felt. This passes deep to the median basilic vein and superficial to the brachial artery. In the past, this provided valuable protection to the artery when workers were injured and bleeding from the median basilic vein. Beyond the middle of the forearm the fleshy parts of the superficial flexor muscles cease, and only the tendons remain, so that the limb narrows rapidly.

The primary flexor of the elbow is the brachialis muscle which lies undernieth the biceps. The biceps muscle causes flexion as well as supination (outward forearm rotation). Pronation is caused by flexion of the pronator teres and pronator quadratus muscles of the forearm. The primary extensor of the elbow is the triceps muscle.

The shoulder joint is composed of three bones: the clavicle (collarbone), the scapula (shoulder blade), and the humerus (upper arm bone) (see diagram). Two joints facilitate shoulder movement. The acromioclavicular (AC) joint is located between the acromion (part of the scapula that forms the highest point of the shoulder) and the clavicle.

The glenohumeral joint, to which the generic term "shoulder joint" usually refers, is a ball-and-socket joint that allows the arm to rotate in a circular fashion or to hinge out and up away from the body. (The "ball" is the top, rounded portion of the upper arm bone or humerus; the "socket," or glenoid, is a dish-shaped part of the outer edge of the scapula into which the ball fits.) Arm movement is further facilitated by the ability of the scapula to slide both laterally and vertically along the rib cage. The capsule is a soft tissue envelope that encircles the glenohumeral joint. It is lined by a thin, smooth synovial membrane.

The bones of the shoulder are held in place by muscles, tendons, and ligaments.

Tendons are tough cords of tissue that attach the shoulder muscles to bone and assist the muscles in moving the shoulder. Ligaments attach shoulder bones to each other, providing stability.

For example, the front of the joint capsule is anchored by three glenohumeral ligaments.

The rotator cuff is a structure composed of tendons that, with associated muscles, holds the ball at the top of the humerus in the glenoid socket and provides mobility and strength to the shoulder joint.

Two filmy sac-like structures called bursae permit smooth gliding between bone, muscle, and tendon. They cushion and protect the rotator cuff from the bony arch of the acromion.

The muscular system in vertebrates consists of three different types of muscles: cardiac, skeletal and smooth. Cardiac muscle is a striated muscle that makes up the heart. It is the only type of muscle consisting of branching fibers.

Skeletal muscle consists of voluntary muscles attached to the frame of the skeletal system enabling bodily movement.

Smooth muscle is the involuntary muscle that enables the movement of internal organs. Movement of all muscles is controlled through the nervous system.

Muscle is a contractile form of tissue. It is one of the four major tissue types, the other three being epithelium, connective tissue and nervous tissue. Muscle contraction is used to move parts of the body, as well as to move substances within the body.

Types

There are three general types of muscle. The first two are "striated", they contain sarcomeres; the third type is "smooth":

Striated muscles contain sarcomeres

cardiac muscle: found within the heart

skeletal muscle (or "voluntary"): attached to the skeleton and used to facilitate movement Smooth muscles do not contain sarcomeres smooth muscle (or "involuntary"): found within structures such as the intestines, throat and blood vessels.

The differences in characteristics of the smooth muscles and the striated muscles include: the fibers of the smooth muscles are not arranged regularly as the ones of striated muscles, smooth muscles are use to sustain longer contraction or even near permanent whereas the striated muscles are often used for short, burst activities.

Muscles within the skeletal muscle are also divided into two subtypes:

Slow twitch (type I or "red") - rich in myoglobin (which is red and carries oxygen), higher aerobic metabolism and mitochondria and hence more capable of endurance activities (activities that don't require maximum strength).

Fast twitch (type II) - more anaerobic metabolism (due to less myoglobin and mitochondria) but better at generating more power in short bursts (at the cost of quicker fatiguability). Type II fibers are used when a task requires more than 25% of your strength.

Type II fibers are further divided into two sub-categories :

type IIx fibers : they are the biggest and strongest, but can't sustain effort for more than a few seconds. They're also called the couch-potato fibers, because when a person excercises regularly, type IIx fibers tend to become type IIa fibers (at least a fraction of them does). Thus, sedentary people have a higher proportion of type IIx fibers.

type IIa fibers : are also used for strength-and-power activities, but can sustain an effort longer than the type IIx fibers can (for up to 3 minutes in highly trained athletes).

Anatomy

Muscle is composed of muscle cells (sometimes known as "muscle fibers"). Within the cells are myofibrils; myofibrils contain sarcomeres, which are composed of actin and myosin. Individual muscle cells are lined with endomysium. Muscle cells are bound together by perimysium into bundles called fascicules; the bundles are then grouped together to form muscle, which is lined by epimysium. Muscle spindles are distributed throughout the muscles and provide feedback sensory information to the central nervous system.

Skeletal muscle is arranged in discrete groups, examples of which include the biceps brachii. It is connected by tendons to processes of the skeleton. In contrast, smooth muscle occurs at various scales in almost every organ, from the skin (in which it controls erection of body hair) to the blood vessels and digestive tract (in which it controls the caliber of a lumen and peristalsis).

The three types of muscle have significant differences, but all use the movement of actin against myosin to produce contraction and relaxation. In skeletal muscle, contraction is stimulated by electrical impulses transmitted by the nerves, the motor nerves and motoneurons in particular. All skeletal muscle and many smooth muscle contractions are facilitated by the neurotransmitter acetylcholine.

Muscles and muscular activity account for most of the body's energy consumption. Muscles store energy for their own use in the form of glycogen, which represents about 1% of their mass. This can be rapidly converted to glucose when more energy is necessary.

Nervous control

Efferent leg

Vertebrates move muscles in response to voluntary and autonomic signals from the brain. Deep muscles, superficial muscles, muscles of the face and internal muscles all correspond with dedicated regions in the brain.

In addition, muscles react to reflexive nerve stimuli that do not always send signals all the way to the brain, but most muscle activity is the result of complex interactions between various areas of the brain.

Nerves that control skeletal muscles in mammals correspond with neuron groups along the primary motor area of the brain's cerebral cortex. Commands are routed though the basal ganglia and are modified by input from the cerebellum before being relayed through the pyramidal tract to the spinal cord and from there to the motor end plate at the muscles. Along the way, feedback loops such as that of the extrapyramidal system contribute signals to influence muscle tone and response.

Deeper muscles such as those involved in posture often are controlled from nuclei in the brain stem and basal ganglia.

Afferent leg

Sometimes known as muscle memory, the sense of where our bodies are in space is called proprioception, the perception of body awareness. More easily demonstrated than explained, proprioception is the "unconscious" awareness of where the various regions of the body are located at any one time. This can be demonstrated by anyone closing their eyes and waving their hand around. Assuming proper proprioceptive function, at no time will the person lose awareness of where the hand actually is, even though it is not being detected by any of the other senses.

Several areas in the brain coordinate movement and position with the feedback information gained from proprioception. The cerebellum and nucleus ruber in particular continuously sample position against movement and make minor corrections to assure a smooth projection.

Role in health and disease

Exercise

Exercise is often recommended as a means of improving motor skills, fitness and muscle strength. Exercise has several effects upon muscles, connective tissue and bone, and the nerves that stimulate the muscles.

Disease

Symptoms of muscle disease may include weakness or spasticity/rigidity, myoclonus (twitching) and myalgia (muscle pain). Diagnostic procedures that may reveal muscular disorders include testing creatine kinase levels in the blood and electromyography (measuring electrical activity in muscles).

Neuromuscular diseases are those that affect the muscles and/or their nervous control. In general, problems with nervous control can cause spasticity or paralysis, depending on the location and nature of the problem. A large proportion of neurological disorders leads to problems with movement, ranging from cerebrovascular accident (stroke) and Parkinson's disease to Creutzfeldt-Jakob disease.

Diseases of the motor end plate include myasthenia gravis, a form of muscle weakness due to antibodies to the acetylcholine receptor, and its related condition Lambert-Eaton myasthenic syndrome (LEMS). Tetanus and botulism are bacterial infections in which bacterial toxins cause increased or decreased muscle tone, respectively.

The myopathies are all diseases affecting the muscle itself, rather than its nervous control.

Muscular dystrophy is a large group of diseases, many of them hereditary, where the muscle integrity is disrupted. It leads to progressive loss of strength, high dependence and decreased life span.

Inflammatory muscle disorders:

Polymyalgia rheumatica (or "muscle rheumatism") is an inflammatory condition that mainly occurs in the elderly; it is associated with giant-cell arteritis. It often responds dramatically to glucocorticoids (e.g. prednisolone).

Polymyositis, dermatomyositis and inclusion-body myositis are autoimmune conditions in which the muscle is affected.

Rhabdomyolysis is the breakdown of muscular tissue due to any cause. While it may not lead to any muscular symptoms at all, the myoglobin thus released may cause acute renal failure.

Tumors of muscle include:

Smooth muscle: leiomyoma (benign, very common in the uterus), leiomyosarcoma (malignant, very rare)

Striated muscle: rhabdomyoma (benign) and rhabdomyosarcoma (malignant) - both very rare

Metastasis from elsewhere (e.g. lung cancer)

Smooth muscle has been implicated to play a role in a large number of diseases affecting blood vessels, the respiratory tract (e.g. asthma), the digestive system (e.g. irritable bowel syndrome) and the urinary tract (e.g. urinary incontinence). These disease processes are not usually confined to the muscular tissue.

The strongest human muscle

Depending on what definition of "strongest" is used, many different muscles in the human body can be characterized as being the "strongest."

In ordinary parlance, muscular "strength" usually refers to the ability to exert a force on an external object—for example, lifting a weight. By this definition, the masseter or jaw muscle is the strongest. The 1992 Guinness Book of Records records the achievement of a bite strength of 975 lbf (4337 N) for two seconds. What distinguishes the masseter is not anything special about the muscle itself, but its advantage in working against a much shorter lever arm than other muscles.

If "strength" refers to the force exerted by the muscle itself, e.g. on the place where it inserts into a bone, then the strongest muscles are those with the largest cross-sectional area at their belly. This is because the tension exerted by an individual skeletal (striated) muscle fiber does not vary much, either from muscle to muscle, or with length. Each fiber can exert a force on the order of 0.3 micronewtons. By this definition, the strongest muscle of the body is usually said to be the Quadriceps femoris or the Gluteus maximus.

Again taking strength to mean only "force" (in the physicist's sense, and as contrasted with "energy" or "power"), then a shorter muscle will be stronger "pound for pound" (i.e. by weight) than a longer muscle. The uterus may be the strongest muscle by weight in the human body. At the time when an infant is delivered, the human uterus weighs about 40 oz (1.1 kg). During childbirth, the uterus exerts 25 to 100 lbf (100 to 400 N) of downward force with each contraction.

The external muscles of the eye are conspicuously large and strong in relation to the small size and weight of the eyeball. It is frequently said that they are "the strongest muscles for the job they have to do" and are sometimes claimed to be "100 times stronger than they need to be." Eye movements, however, are and probably "need" to be exceptionally fast.

The unexplained statement that "the tongue is the strongest muscle in the body" appears frequently in lists of surprising facts, but it is difficult to find any definition of "strength" that would make this statement true. Note that technically the tongue consists of sixteen muscles, not one. The tongue may possibly be the strongest muscle at birth.

The heart has a claim to being the muscle that performs the largest quantity of physical work in the course of a lifetime. Estimates of the power output of the human heart range from 1 to 5 watts. This is much less than the maximum power output of other muscles; for example, the quadriceps can produce over 100 watts, but only for a few minutes. The heart does its work continously over an entire lifetime without pause, and thus can "outwork" other muscles. An output of one watt continuously for seventy years yields a total work output of 2 to 3 ×109 joules.

In the middle line below the chin can be felt the body of the hyoid bone, just below which is the prominence of the thyroid cartilage called "Adam's apple," better marked in men than in women. Still lower the cricoid cartilage is easily felt, while between this and the suprasternal notch the trachea and isthmus of the thyroid gland may be made out.

At the side the outline of the sterno-mastoid muscle is the most striking mark; it divides the anterior triangle of the neck from the posterior. The upper part of the former contains the submaxillary gland, which lies just below the posterior half of the body of the jaw. The line of the common and the external carotid arteries may be marked by joining the sterno-clavicular articulation to the angle of the jaw. The eleventh or spinal accessory nerve corresponds to a line drawn from a point midway between the angle of the jaw and the mastoid process to the middle of the posterior border of the sterno-mastoid muscle and thence across

the posterior triangle to the deep surface of the trapezius. The external jugular vein can usually be seen through the skin; it runs in a line drawn from the angle of the jaw to the middle of the clavicle, and close to it are some small lymphatic glands. The anterior jugular vein is smaller, and runs down about half an inch from the middle line of the neck. The clavicle or collar-bone forms the lower limit of the neck, and laterally the outward slope of the neck to the shoulder is caused by the trapezius muscle.

Muscles of facial expression

Auricularis anterior muscle

Buccinator muscle

Corrugator supercilii muscle

Depressor anguli oris muscle

Depressor labii inferioris muscle

Depressor septi nasi muscle

Frontalis muscle

Levator anguli oris muscle

Levator labii supeioris muscle

Levator labii superioris alaeque nasi muscle

Levator palpebrae superioris muscle

Mentalis muscle

Nasalis muscle

Occipitalis muscle

Orbicularis oculi muscle

Orbicularis oris muscle

Platysma Procerus muscle

Risorius muscle

Zygomaticus major muscle

Zygomaticus minor muscle

The muscles of mastication

Lateral pterygoid muscle

Masseter muscle

Medial pterygoid muscle

Temporalis muscle

The extraocular muscles

Inferior oblique muscle

Inferior rectus muscle

Lateral rectus muscle

Medial rectus muscle

Superior oblique muscle

Superior rectus muscle

The intraocular muscles

Ciliary muscle

Iris dilator muscle

Iris sphincter muscle

Interspinales muscle

Intertransversarii muscle

Multifidus muscle

Rotatores muscle

Sacrospinalis muscle

Semispinalis muscle

Splenius capitis muscle

Splenius cervicis muscle

The suboccipital muscles

Obliquus capitis inferior muscle

Obliquus capitis superior muscle

Rectus capitis posterior major muscle

Rectus capitis posterior minor muscle

Thorax Muscles

Diaphragm muscle

Intercostales externi muscle

Intercostales interni muscle

Levatores costarum muscle

Serratus posterior inferior muscle

Serratus posterior superior muscle

Subcostales muscle

Transversus thoracis muscle

Abdomen Muscles

Cremaster muscle

Obliquus externus abdominis muscle

Obliquus internus abdominis muscle

Psoas major muscle

Psoas minor muscle

Pyramidalis muscle

Quadratus lumborum muscle

Rectus abdominis muscle

Transversus abdominis muscle

Pelvis Muscles

Coccygeus muscle

Levator Ani muscle

The muscles of the perineum

Bulbospongiosus muscle

Corrugator cutis ani muscle

Ischiocavernosus muscle

Sphincter ani externus muscle

Sphincter ani internus muscle

Sphincter urethrae membranaceae muscle

Transversus perinei profundus muscle

Transversus perinei superficialis muscle

The shoulder complex comprises five individual joints. The term "shoulder" often is used specifically to refer to the principle one of these, the glenohumeral joint, consisting of the proximal ball of the humerus and the associated ("glenoid") socket of the scapula.

Above this lies the acromioclavicular, or "a-c" joint, between the clavicle and the acromial process of the scapula.

The clavicle in turn articulates with the chest wall at the sternoclavicular joint, while the scapula slides across the posterior chest wall via the "scapulothoracic" joints.

The musculature of the shoulder region is similarly complex. The muscles which drive the arm include the pectoralis major (chest muscle), which pulls the arm forward; the trapezius, which elevates the shoulder; the deltoid, which elevates the arm to the side; and the latissimus dorsi, which pulls the arm down.

The clavicular part of the pectoralis major runs downward and outward from the inner half of the clavicle. The clavicular part of the deltoid attaches to the outer third of the clavicle. Between these two muscles is an elongated triangular gap with its base at the clavicle. Here, where the skin is somewhat depressed, the cephalic vein sinks between the two muscles to join the axillary vein. The tip of the coracoid process is situated just under cover of the inner edge of the

deltoid, one inch below the junction between the outer and middle thirds of the clavicle.

The deltoid muscle forms the prominence of the shoulder, and its convex outline is due to the presence of the head of the humerus deep within it. When the humerus is dislocated, the deltoid muscle's appearance becomes flattened.

The pectoralis major forms the anterior fold of the axilla or armpit, the posterior fold being formed by the latissimus dorsi and teres major muscles. The skin of the floor of this space is covered with hair in the adult, and contains many large sweat glands. The axillary vessels and brachial plexus of nerves traverse this region. Below the lateral edge of the pectoralis major, the swelling of the biceps begins to be visible. The course of this large muscle, which flexes the forearm, can easily be traced into its tendon of insertion, in the front of the elbow joint.

Arm

On the sides of the biceps lie the external and internal bicipital furrows, in the latter of which the brachial artery may be felt and compressed. The median nerve lies here, in close relation to the artery. At the bend of the elbow the two condyles of the humerus may be felt, on either side.

The inner one projects beneath the skin, but the outer one is obscured by the rounded outline of the brachioradialis muscle. On the back of the arm lie the three heads of the triceps muscle, the external forming a marked oblique swelling when the forearm is forcibly extended and internally rotated. The triceps serves to "extend", or straighten, the elbow joint.

Elbow

The elbow is composed of three joints, which allow the bones of the elbow to articulate with one another. These bones are the humerus (upper arm), the ulna (medial forearm) and the radius (lateral forearm).

The major articulation of the elbow is the ulno-humeral joint, which enables arm flexion and extension.

Interestingly, a person can perform most daily tasks with a range of flexion from 30 degrees to 70 degrees. Secondly, the head of the radius is dish-shaped and interacts with the capitellum (a spherical structure on the distal humerus) to form the radio-capitellar joint.

This joint is responsible for adding stability to the elbow by transferring forces across the elbow joint. Thirdly, the radial head also spins against the ulna to form the proximal radio-ulnar joint, which is responsible for forearm rotation. The ulnohumeral, radiocapitellar and proximal radioulnar joints work together to position the hand in space.

On the back of the elbow, the olecranon process of the ulna is quite subcutaneous. During extension of the elbow it is in line with the two condyles. Between it and the inner condyle lies the ulnar nerve, here known popularly as the "funny bone". Striking the elbow in this area may send a shock wave of pain down the forearm.

The superficial veins at the bend of the elbow are very conspicuous. Their typical arrangement is an 'M', of which the radial and ulnar veins form the uprights, while the outer oblique bar is the median cephalic and the inner oblique the median basilic vein.

At the divergence of these two the median vein comes up from the front of the forearm, while the two vertical limbs continue up the arm as the cephalic and basilic veins, on the outer and inner sides, respectively. In the upper part of the front of the forearm lies the antecubital fossa or triangle.

Its outer boundary is the brachio-radialis, its inner the pronator radii teres, and its apex the point below at which they merge. In this space are three vertical structures: externally the tendon of the biceps, just internal to this the brachial artery, and still more internally the median nerve.

Coming from the inner side of the biceps tendon the semi-lunar fascia may be felt. This passes deep to the median basilic vein and superficial to the brachial artery. In the past, this provided valuable protection to the artery when workers were injured and bleeding from the median basilic vein. Beyond the middle of the forearm the fleshy parts of the superficial flexor muscles cease, and only the tendons remain, so that the limb narrows rapidly.

The primary flexor of the elbow is the brachialis muscle which lies undernieth the biceps. The biceps muscle causes flexion as well as supination (outward forearm rotation).

Pronation is caused by flexion of the pronator teres and pronator quadratus muscles of the forearm. The primary extensor of the elbow is the triceps muscle.

Hands: Muscles and tendons

The movements of the human hand are accomplished by two sets of each of these tissues. They can be subdivided into two groups: the extrinsic and intrinsic muscle groups. The extrinsic muscle groups are the long flexors and extensors. They are called extrinsic because the muscle belly is located on the forearm.

Intrinsic hand muscles

The Intrinsic muscle groups are the thenar and hypothenar muscles (thenar referring to the thumb, hypothenar to the little finger), the interosseus muscles (between the metacarpal bones, four dorsally and three volarly) and the lumbrical muscles.

These muscles arise from the deep flexor (and are special because they have no bony origin) and insert on the dorsal extensor hood mechanism.

The extrinsic muscles of the hand

The flexors

The fingers have two long flexors, located on the underside of the forearm. They

insert by tendons to the phalanges of the fingers. The deep flexor attaches to the distal phalanx, and the superficial flexor attaches to the middle phalanx. The flexors allow for the actual bending of the fingers.

The thumb has one long flexor and a short flexor in the thenar muscle group. The human thumb also has other muscles in the thenar group (opponens- and abductor muscle), moving the thumb in opposition, making grasping possible.

The extensors

Located on the back of the forearm and a connected in a more complex way then the flexors to the dorsum of the fingers. The tendons unite with the interosseous and lumbrical muscles to form the extensorhood mechanism. The primary function of the extensors is to straighten out the digits. The thumb has two extensors in the forearm; the tendons of these form the anatomical snuff box. Also, the index finger and the little finger have an extra extensor, used for instance for pointing.

Muscles of the thigh

Anterior compartment of the thigh

Quadriceps femoris, which is composed of:

Vastus lateralis

Vastus medialis

Vastus intermedius

Rectus femoris

Sartorius

Adductor longus

Adductor brevis

Adductor magnus

Gracilic

Pectineus

Gluteus maximus

Tensor fascia lata

Posterior compartment of the thigh

Biceps femoris

Semimembranosus

Semitendinosus

Muscles of the calf

The muscles of the human lower

legPopliteus

The anterior compartment

Tibialis anterior

Extensor digitorum longus

Extensor hallicus longus

Peroneus tertius

The posterior compartment

Gastrocnemius (attached to the calcaneus by Achilles' tendon)

Plantaris

Soleus

The lateral compartment

Peroneus longus

Peronius brevis

The deep posterior compartment

Tibialis posterior

Flexor digitorum longus

Flexor hallicus longus

Vasculature of the leg

The arteries

Common femoral artery

Deep femoral artery

Superficial femoral artery

Popliteal artery

Anterior tibial artery

Posterior tibial artery

Peroneal artery

Arcuate artery

The veins

Greater saphenous vein

Lesser saphenous vein

Femoral vein

Popliteal vein

Anterior tibial vein

Posterior tibial vein

Peroneal vein

The muscles of the foot

abductor hallucis muscle

adductor hallucis muscle

dorsal interossei muscles

flexor hallucis brevis muscle

flexor digitorum brevis muscle

plantar interossei muscles

quadratus plantae muscle

lumbrical muscles

In the human body, the heart is normally situated slightly to the left of the middle of the thorax, underneath the sternum (breastbone). It is enclosed by a sac known as the pericardium and is surrounded by the lungs. In adults, it weighs about 300-350 g. It consists of four chambers, the two upper atria (singular: atrium) and the two lower ventricles.

A thick muscular wall, the septum, divides the right atrium and ventricle from the left atrium and ventricle, preventing blood from passing between them. Valves between the atria and ventricles (atrioventricular valves) maintain coordinated unidirectional flow of blood from the atria to the ventricles.

The left ventricle pumps blood throughout the body's arteries and veins; the right ventricle pumps blood to the lungs. Compared with the walls of the atria, the ventricle walls are thicker.

Anterior (frontal) view of the opened heart. White arrows indicate normal blood flow. (SVG version)Oxygen-depleted or deoxygenated blood from the body enters the right atrium through two great veins, the superior vena cava which drains the upper part of the body and the inferior vena cava that drains the lower part. The blood then passes through the tricuspid valve to the right ventricle.

The right ventricle pumps the deoxygenated blood to the lungs, through the pulmonary artery. In the lungs gaseous exchange takes places and the blood releases carbon dioxide into the lung cavity and picks up oxygen. The oxygenated blood then flows through pulmonary veins to the left atrium.

From the left atrium this newly oxygenated blood passes through the mitral valve to enter the left ventricle. The left ventricle then pumps the blood through the aorta to the entire body except the lungs.

The left ventricle is much more muscular than the right as it has to pump blood around the entire body, which involves exerting a considerable force to overcome the vascular pressure. As the right ventricle needs to pump blood only to the lungs, it requires less muscle.

Even though the ventricles lie below the atria, the two vessels through which the blood exits the heart (the pulmonary artery and the aorta) leave the heart at its top side.

The contractile nature of the heart is due to the presence of cardiac muscle in its wall which can work continuously without fatigue. The heart wall is made of three distinct layers.

The first is the outer epicardium which is composed of a layer of flattened epithelial cells and connective tissue. Beneath this is a much thicker myocardium made up of cardiac muscle. The endocardium is a further layer of flattened epithelial cells and connective tissue which lines the chambers of the heart.

The blood supply to the heart itself is supplied by the left and right coronary arteries, which branch off from the aorta.

The cardiac cycle

Atrial systole

Ventricular systoleThe function of the heart is to pump blood around the body. Every single beat of the heart involves a sequence of events known as the cardiac cycle, which consists of three major stages: atrial systole, ventricular systole and complete cardiac diastole. The atrial systole consists of the contraction of the atria and the corresponding influx of blood into the ventricles.

Once the blood has fully left the atria, the atrioventricular valves, which are situated between the atria and ventricular chambers, close. This prevents any backflow into the atria. It is the closing of the valves that produces the familiar beating sounds of the heart, commonly referred to as the "lub-dub" sound.

The ventricular systole consists of the contraction of the ventricles and flow of blood into the circulatory system. Again, once all the blood empties from the ventricles, the pulmonary and aortic semilunar valves close. Finally complete cardiac diastole involves relaxation of the atria and ventricles in preparation for refilling with circulating blood.

Regulation of the cardiac cycle

Cardiac muscle is myogenic, which means that it is self-exciting. This is in contrast with skeletal muscle, which requires either conscious or reflex nervous stimuli. The heart's rhythmic contractions occur spontaneously, although the frequency or heart rate can be changed by nervous or hormonal influences such as exercise or the perception of danger.

The rhythmic sequence of contractions is coordinated by the sinoatrial and atrioventricular nodes. The sinoatrial node, often known as the cardiac pacemaker, is located in the upper wall of the right atrium and is responsible for the wave of electrical stimulation (See action potential) that initiates atria contraction. Once the wave reaches the atrioventricular node, situated in that separates the ventricular chambers, it is conducted through the bundles of His and causes contraction of the ventricles.

The time taken for the wave to reach this node from the sinoatrial nerve creates a delay between contraction of the two chambers and ensures that each contraction is coordinated simultaneously throughout all of the heart.

In the event of severe pathology, the Purkinje fibers can also act as a pacemaker; this is usually not the case because their rate of spontaneous firing is considerably lower than that of the other pacemakers and hence is overridden.

Following are some basic functions of the human circulatory system:

Delivery of oxygen and nutrients to all parts of the body. Collection of metabolic wastes and delivery to the excretory organs, e.g. kidneys. Role in the immune system of defense against infection.

Transport of hormones.

The human circulatory system is comprised of the blood, the vascular system, and the heart.

The heart is the muscular organ which pumps the blood. The vascular system is made up of arteries, veins, and capillaries. Arteries are blood vessels that carry blood away

from the heart. Veins are blood vessels that return blood to the heart. Capillaries are the smallest blood vessels, and are where the exchange of nutrients and gases takes place between the red blood cells and the body tissues.

Humans have a double circulatory system which consists of separate but connecting circulations: the pulmonary circulation and the systemic circulation.

Pulmonary circulation

The right ventricle pumps deoxygenated blood into the pulmonary arteries. These arteries carry the blood to the lungs, where it passes through a capillary network close to air-filled alveoli.

This enables the release of carbon dioxide and the uptake of oxygen from the air. The now oxygenated blood returns to the left atrium in the pulmonary veins.

Systemic circulation

Oxygenated blood from the lungs returns to the heart via the pulmonary veins, flows into the left atrium and then into the left ventricle, which then pumps the blood through the aorta, the major artery which supplies blood to the body.

Smaller arteries branch off the aorta.

Splanchnic circulation

Also called visceral circulation, the splanchnic circulation is the part of the systemic circulation that supplies the digestive organs.

The major arteries of the splanchnic circulation branch directly off the aorta and include the celiac artery (celiac axis), superior mesenteric artery, and inferior mesenteric artery.

Portal circulation

There are two exceptions to the system of double circulation.

The deoxygenated blood from the capillaries of the gastrointestinal tract drains into the portal vein which, instead of going directly back to the heart, leads to the liver.

This allows the liver to take up the nutrients that were extracted by the intestines from food. The liver also neutralizes some toxins taken up by the intestines. Blood from the liver drains via the hepatic veins into the inferior vena cava and then the right side of the heart.

There is also a small portal flow from the hypothalamus to the anterior pituitary gland.

Fetal circulation

The circulatory system of the fetus is different, as the fetus does not use its lungs yet and obtains oxygen and nutrients from the placenta through the umbilical cord. After birth, the fetal circulatory system undergoes several anatomical changes, including closure of the ductus arteriosus and foramen ovale.

History of discovery

The valves of the heart were discovered by a physician of the Hippocratean school around the 4th century BC. However their function was not properly understood then. Because blood pools in the veins after death, arteries look empty. Ancient anatomists assumed they were filled with air and that they were for transport of air.

Herophilus distinguished veins from arteries but thought that the pulse was a property of arteries themselves. Erasistratus observed that arteries that were cut during life bleed. He ascribed the fact to the phenomenon that air escaping from an artery is replaced with blood that entered by very small vessels between veins and arteries. Thus he apparently postulated capillaries but with reversed flow of blood.

Galen in the 2nd century AD knew that blood vessels carry blood and identified venous (dark red) and arterial (brighter and thinner) blood, each with distinct and separate functions. Growth and energy were derived from venous blood created in the liver from chyle, while arterial blood gave vitality by containing pneuma (air) and originated in the heart.

Blood flowed from both creating organs to all parts of the body where it was consumed and there was no return of blood to the heart or liver. The heart did not pump blood around, the heart's motion sucked blood in during diastole and the blood moved by the pulsation of the arteries themselves.

Galen believed that the arterial blood was created by venous blood passing from the left ventricle to the right by passing through 'pores' in the interventricular septum, air passed from the lungs via the pulmonary artery to the left side of the heart.

As the arterial blood was created 'sooty' vapors were created and passed to the lungs also via the pulmonary artery to be exhaled.

Ibn Nafis in 1242 was the first person to accurately describe the process of blood circulation in the human body. Contemporary drawings of this process have survived. In 1552 Servetus described the same and Realdo Colombo proved the concept. All these results were not widely accepted however.

Finally William Harvey, a pupil of Hieronymus Fabricius (who had earlier described the valves of the veins without recognizing their function), performed a sequence of experiments and announced in 1628 the discovery of the human circulatory system as his own and published an influential book about it.

This work with its essentially correct exposition slowly convinced the medical world. Harvey was not able to identify the capillary system connecting arteries and veins; these were later described by Marcello Malpighi.