NINTH EDITION LAST'S ANATOMY REGIONAL AND APPLIED Churchill Livingstone 1. Introduction to regional anatomy PART 1 TISSUES AND STRUCTURES The body is composed of four basic tissues — epithe lium, connective tissue, muscle and nerve—and every part of the body that is examined with either the naked eye or the microscope can only be made up of one or more of these four elements. There are of course different kinds of these tissues depending on the differing functional requirements of the various organs and parts of the body, and the details of cell types and intercellular substances are dealt with in texts on histology. This chapter brings together some notes on certain body tissues and structures to form a relevant introduction to regional anatomy. Fig. 1.1 Sections of thick skin, on the left, and thin skin on the right. The definition of thick and thin depends on the thickness of the keratinized layer; the overall depth of the combined epidermis and dermis is the same. SKIN Skin consists of two elements: epithelium and connec tive tissue. The epithelium of the skin, which is given the special name of epidermis, is of the stratified squamous keratinizing variety. The various skin appendages—sebaceous glands, sweat glands, nails and hair — are specialized derivatives of this epidermis, which is ectodermal in origin. The connective tissue part of the skin, which is mesodermal in origin, is the dermis, consisting mainly of bundles of collagen fibres together with some elastic tissue, blood vessels, lymphatics and nerve fibres, all embedded in ground substance. When dried the dermis makes greenhide; when tanned it makes leather. The uppermost layer of the epidermis is the stratum corneum (Fig. 1.1), the cornified or horny layer, consisting of dead cells (keratin) that have lost their nuclei and that are constantly being rubbed off and replaced by cells moving up from deeper layers. In the scalp scaly flakes of the horny layer may be trapped by hairs instead of falling off invisibly, so forming dandruff. The horny layer is normally softened by the greasy secretion of sebaceous glands and moistened somewhat by the watery secretion of the sweat glands. Fat solvents or emulsifiers remove the grease and leave the horny layer stiff and harsh. Undue contact with water macer ates the keratin which, by imbibition, becomes thick, soft and white ('washerwoman's fingers'). The terms thick skin and thin skin refer to the thick ness of the cornified layer (Fig. 1.1). In thick skin, such as on the sole of the foot, the stratum corneum is thick and paradoxically the dermis is relatively thin. In thin skin such as on the front of the forearm the stratum corneum is thin but the dermis relatively thick. While partly due to its thickness and blood flow, the main factor determining the colour of skin is the degree of pigmentation produced by melanocytes which are mainly found in the basal layer of the epidermis. These cells manufacture melanin granules which are liberated from the melanocytes and ingested by other epidermal cells; thus the presence of melanin granules within a cell does not necessarily imply that the cell has 1 2 LAST S ANATOMY manufactured them. The synthesizing cells can be distinguished from the others by the dopa reaction, in which DihydrOxyPhenylAlanine is converted by the enzyme tryosinase into melanin. The differences in skin colour between the light- and dark-skinned races are not due to differences in melanocyte numbers, for these are similar in all; in the darker skins the melanocytes are more active and thus produce more pigment, but there are also racial differences between melanins which can vary in colour from yellows to browns and blacks. White skin becomes tanned by the sun because ultra violet light stimulates melanocyte activity (and may also induce cancerous change, especially in the fairest skins that become sunburnt). The skin is bound down to underlying structures to a variable extent. On the dorsum of the hand or foot it can be pinched up and moved readily. On the palm and sole this is impossible, for here the dermis is bound firmly to the underlying aponeurosis, a necessary functional requirement to improve the grip of the hand and foot. The creases in the skin are flexure lines over joints. The skin always folds in the same place. Along these flexure lines the skin is thinner and is bound more firmly to the underlying structures (usually deep fascia). The site of the flexure lines does not always correspond exactly to the topography of the underlying structures. For example, the anterior flexure line for the hip lies below the inguinal ligament; the posterior flexure line is not influenced by the oblique lower border of gluteus maximus and lies horizontally, to make the fold of the buttock. Skin contains about 3 million sweat glands, which are distributed all over the skin except on the margins of the lips, glans penis and tympanic membranes. The greatest concentration is in the thick skin of the palms and soles, and on the face including the forehead. Structurally they resemble coiled test-tubes that extend below the dermis into the subcutaneous tissue, with ducts that are reasonably straight as they pass through the dermis but which pursue a spiral, corkscrew-like course through the epidermis. There are two types of gland — eccrine and apocrine — although the mode of secretion is the same in both. The vast majority are eccrine glands whose purpose is to deliver water to the body surface and so assist in temperature regulation, although those on the palms, soles and forehead respond to emotional stress — as is well known to students in oral examinations! The apocrine glands are larger and confined to the axillae, areolae of the breasts and urogenital regions (breasts themselves are modified apocrine glands). They correspond to the scent glands of animals where they are important for recognition in the breeding season, and even in humans they are under the control of sex hormones, becoming active at puberty. Their ducts, like those of sebaceous glands (see below), open into hair follicles. Although human apocrine secretion is odourless, it may make its presence felt because of the action of skin surface bacteria which degrade it into less appreciated by-products. Sebaceous glands are usually confined to hairy skin where they open by a very short duct into the side of a hair follicle; only in a few sites do they open directly on to the skin surface—eyelids, lips, papillae of the breasts and labia minora. There are none on the palms or soles. Structurally the glands form a grape-like cluster beside a hair follicle. Hair: the keratin of the skin surface is soft keratin, but hair and nails are a hard type of keratin. There is no new development of hair follicles after birth. Each hair is formed from the hair matrix, a region of epidermal cells at the base of the hair follicle. As the cells move up inside the tubular epidermal sheath of the follicle they soon lose their nuclei and become converted into the hard keratin rod that is the hair. Melanocytes in the hair matrix impart pigment to the hair cells. The differing colours of hair are due to a mixture of three kinds of pigment. The change with age is due to decreasing melanocyte activity. Most follicles have an arrector pili muscle attached to the connective tissue of the base of the follicle and passing obliquely to the upper part of the dermis. Composed of smooth muscle with a sympa thetic innervation, it is on the same side of the follicle as the sebaceous gland, so that when contracting to make the hair 'stand on end', it may also squeeze the gland. Hair follicles do not grow continually, and growth periods vary in different sites. Those on the scalp have a growing phase of 2-3 years followed by a few months in a resting phase, during which time the hair becomes detached from the base of the follicle and falls out before the matrix starts to make a new hair. Adjacent follicles are out of phase with one another so that the constant renewal and replacement are not obvious. In the eyebrows the growing phase is only a month or two, with a longer resting period. Nails: a finger- or toe-nail is formed from a nail matrix, a similar epidermal specialization to hair matrices. Surface area: in skin damage by burns, an estimate of the affected surface area is important in assessing the need for fluid replacement therapy, and the 'Rule of Nines' gives a guide to the size of body parts in propor tion to the whole: head 9%; upper limb 9%; lower limb 18%; front of thorax and abdomen 18%; back of thorax and abdomen 18%. Tension lines and wrinkle lines of skin, due to the pattern of fibre bundles in the dermis, run as indicated INTRODUCTION TO REGIONAL ANATOMY Fig. 1.2 Tension lines of the skin, front and back. in Figure 1.2. Skin creases such as those near joints run parallel with tension lines, but they and the wrinkles of ageing do not necessarily correspond to the cleavage lines originally described by Langer in 1861. Incisions made along creases and wrinkle lines heal with a minimum of scarring; hence incisions should preferably not be made across these lines, and certainly never across the flexure creases on the flexor surfaces of joints. SUBCUTANEOUS TISSUE The skin is connected to the underlying bones or deep fascia by a layer of areolar tissue that varies widely in character in different species. In hairy mammals it is loose and tenuous with a minimum of fat, so that it is a simple matter to skin the animal. In others, including man, fat is plentiful and fibrous bands in the fat tether the skin to the deep fascia. Such an animal is more difficult to skin; it has a blanket of fat beneath the skin, called the panniculus adiposus. The panniculus adiposus is well developed in man, and in it nerves, blood vessels and lymphatics pass to the skin. The panniculus adiposus is a substitute for a fur coat in 'hairless' mammals (e.g. man, pig, cetacea). The term superficial fascia is so ingrained in nomen clature that there is no hope of discarding it. Yet the tissue bears no possible resemblance to the other so- called 'deep5 fasciae, and the names panniculus adiposus, subcutaneous tissue or subcutaneous fat are greatly to be preferred. In the panniculus adiposus are flat sheets of muscle, the panniculus carnosus. The degree of their develop ment varies widely in different animals. In domestic quadrupeds such as sheep and horses the sheet is present over most of the body wall. It can be seen on the carcass in a butcher's shop, lying on the surface of the fat, generally incised in parallel slits to make an attractive pattern. It can be seen in action when a horse twitches the skin over its withers. The essential point about the panniculus carnosus is that one end of each muscle fibre is attached to the skin, the other end being usually attached to deep fascia or bone. In man the sheet is well developed and highly differ entiated to form the muscles of the scalp and face including the platysma, and remnants persist in such subcutaneous muscles as the palmaris brevis and as unstriped muscle in the corrugator cutis ani, in the dartos sheet of the scrotum and in the subareolar muscle of the nipple. DEEP FASCIA The limbs and body wall are wrapped in a membrane of fibrous tissue, the deep fascia. It varies widely in thick ness. In the iliotibial tract of the fascia lata, for example, it is very well developed, while over the rectus sheath and external oblique aponeurosis of the abdominal wall it is so thin as to be scarcely demonstrable and is usually considered to be absent. In other parts, such as the face and the ischioanal fossa, it is entirely absent. A feature of the deep fascia of the body and limbs is that it never passes freely over bone but is always anchored firmly to the periosteum. A pin thrust into a muscle will pass through skin, subcutaneous tissue and deep fascia; one thrust into a subcutaneous bone will pass through skin, panniculus adiposus and periosteum only (Fig. 1.3). The deep fascia serves for attachment of the skin by way of fibrous strands in the subcutaneous tissue. In a few places it gives attachment to underlying muscles, but almost everywhere in the body the muscles are free to glide beneath it as they lengthen and shorten. Deep fascia is very sensitive. Its nerve supply, and that of subcutaneous periosteum where no deep fascia exists, is that of the overlying skin. The nerves to muscles do not supply the investing layer of deep fascia, but only the fibrous tissue of deep intermuscular spaces, and deep periosteum. As well as the investing layer of deep fascia on the surface of the body there are many other fascial layers m deeper parts, of widely differing character. In general it 4 LAST'S ANATOMY Deep fascia Periosteum aponeuroses the histological structure is the same. Tendons have a blood supply from vessels which descend from the muscle belly and anastomose with vessels ascending from the periosteum at the bony attachment. In long tendons intermediate vessels from a neighbouring artery reinforce the longitudinal anasto- Fig. 1.3 Deep fascia blending with periosteum in a transverse section of the tibia. may be said that where fasciae lie over non-expansile parts (e.g. muscles of the pelvic wall, prevertebral muscles) they are well developed membranes readily demonstrable, able to be sutured after incision; but where they lie over expansile parts (e.g. muscles of the pelvic floor, cheek, pharynx) they are indefinite and thin collections of loose areolar tissue, nothing more nor less than the epimysium of the underlying muscle LIGAMENTS Ligaments are composed of dense connective tissue, mainly collagen fibres (white fibrous tissue) and they attach bone to bone. They have the physical property of being non-elastic and unstretchable. Only if subjected to prolonged strain will collagen fibres elongate, and undue mobility is then possible in the joints (e.g. in flat feet, contortionists). White fibrous tissue ligaments are so arranged that they are never subjected to prolonged strain, with the curious exception of the sacroiliac ligaments and the intervertebral discs, which are never free from the strain of the whole weight of the body except in recumbency. A second type of ligament is composed of elastic tissue, which regains its former length after stretching. It is yellow in colour, hence the name of the ligamenta flava between the laminae of the vertebrae. The capsular ligaments of the joints of the auditory ossicles are made of yellow elastic tissue. Synovial sheaths Where tendons bear heavily on adjacent structures, and especially where they pass around loops or pulleys of fibrous tissue or bone which change the direction of their pull, they are lubricated by being provided with a synovial sheath. The parietal layer of the sheath is firmly attached to the surrounding structures, the visceral layer is firmly fixed to the tendon, and the two layers glide on each other, lubricated by a thin film of synovial fluid secreted by the lining cells of the sheath. The visceral and parietal layers join each other. Usually they do not enclose the tendon cylindrically; it is as though the tendon were pushed into the double layers of the closed sheath from one side (Fig. 1.4). In this way blood vessels can enter the tendon to reinforce the longitudinal anastomosis. In other cases blood vessels perforate the sheath and raise up a synovial fold like a little mesentery—a mesotendon or vinculum — as in the flexor tendons of the digits (Fig. 2.54C, p. 118). RAPHES A raphe is an interdigitation of the short tendinous ends of fibres of flat muscle sheets. It can be elongated passively by separation of its attached ends. There is, for example, no such structure as a pterygomandibular ligament; if there were, the mandible would be fixed, since ligaments do not stretch. The buccinator and superior constrictor interdigitate in the pterygo mandibular raphe, the length of which varies with the position of the mandible. The mylohyoid raphe, pharyngeal raphe and anococcygeal raphe are further examples. TENDONS Tendons have a similar structure to collagenous ligaments, and attach muscle to bone. They may be cylindrical or flat; even if flattened into sheet-like Fig. 1.4 Arterial supply to a long tendon, with the vessel passing through a gap in the synovial sheath. INTRODUCTION TO REGIONAL ANATOMY 5 CARTILAGE Cartilage is a type of dense connective tissue in which cells and fibres are embedded in a firm ground substance or matrix, and there are three types. The commonest is hyaline cartilage, which covers the articular surfaces of typical synovial joints and forms the epiphyseal growth plates of growing bones. The usual histological sections of cartilage give no idea of the amount of collagen fibres that it contains, because the fibres and ground substance both have the same refractive index. Unlike bone, cartilage is avascular and capable of a small amount of deformation; it therefore has a certain resistance to fracture, but when it is damaged it is usually repaired by the formation of fibrous tissue, not new cartilage. However, there are circumstances in which new cartilage can develop, not from surviving cartilage cells but by the differentiation of 'osteochondrogenic' cells—the name sometimes now given to cells of the osteogenic layer of the periosteum which appear capable of becoming osteoblasts or chondroblasts depending on the vascularity of the area (p. 9). Fibrocartilage resembles ligament and tendon but contains small islands of cartilage cells and ground substance between the collagen bundles. It is found in intervertebral discs and the discs or disc-like structures of some joints, such as the labrum of the shoulder and hip joints and the menisci of the knee joint. It also occurs on the articular surfaces of the clavicle and mandible, and at those attachment sites of tendon to bone epiphyses, which leave a smooth marking on the bone. Both hyaline cartilage and fibrocartilage tend to calcify and even ossify in old age. The third type, elastic cartilage (epiglottis and pinna), has a ground substance that contains large numbers of elastic fibres. It can be easily distorted, and just as easily springs back to its original shape when at rest. It is functionally ideal for the skeletal framework of the human pinna, the auditory tube and the epiglottis. It never calcifies or ossifies. Fibrocartilage has an ordinary blood supply (rather sparse because its metabolic rate is low) but hyaline and elastic cartilage have no capillaries, their cells being nourished by diffusion through the ground substance. MUSCLE There are three kinds of muscle—skeletal, cardiac and visceral—although the basic histological classification is into two types — striated and non-striated. This is because both skeletal and cardiac muscle are striated, a structural characteristic due to the way the filaments of actin and myosin are arranged. The term striated muscle is usually taken to mean skeletal muscle. Visceral muscle is non-striated and so is usually called smooth muscle (although it too contains filaments of actin and myosin, but they are arranged differently). The terms 'muscle cell' and 'muscle fibre' are the same thing. Smooth and cardiac muscle fibres, like most cells, usually have a single nucleus, but skeletal muscle fibres are multinucleated cells; in fibres that are several centimetres long there are thousands of nuclei, charac teristically situated just beneath the cell membrane. Smooth muscle consists of narrow spindle-shaped cells usually lying parallel. In tubes that undergo peristalsis they are arranged in longitudinal and circular fashion (as in the alimentary canal and ureter). In viscera that undergo a mass contraction without peristalsis (such as urinary bladder and uterus) the fibres are arranged in whorls and spirals rather than demonstrable layers. Contractile impulses are trans mitted from one cell to another at sites called nexuses or gap junctions, where adjacent cell membranes lie unusu ally close together. Innervation is by autonomic nerves and because of the gap junctions many muscle fibres do not receive nerve fibres. Cardiac muscle consists of much broader, shorter cells that branch. Part of the boundary membranes of adjacent cells make very elaborate interdigitations with one another (at the 'intercalated discs' of light microscopy) to increase the surface area for impulse conduction. The cells are arranged in whorls and spirals; each chamber of the heart empties by mass contraction, not peristalsis. Innervation, like that of visceral muscle, is by autonomic nerves. Skeletal muscle (the red meat of the butcher) consists of non-branching fibres bound together by loose areolar tissue containing the usual complement of cells such as fibroblasts and macrophages. This connec tive tissue is condensed like the skin of a sausage on the surface of all muscles, forming a membrane of varying thickness and density well known to every dissector; it is the material dissected away and discarded in the process of 'cleaning' a muscle for demonstration purposes. The membranous envelope, or epimysiurn, is impervious to the spread of fluid such as pus. It is seldom of such a nature as to warrant special descrip tion as a named fascia, but one such in the neck is the buccopharyngeal fascia (p. 488). Skeletal muscle fibres can be shown by histochemical and other means to be of two types, often called red and white, and, while in animals there are some whole skeletal muscles which consist of a single type, all human muscles are a mixture of both types and all appear red, although one type may predominate. The 6 last's anatomy red fibres, slow twitch in physiological terms with aerobic respiration, have a high content of mitochon dria, myoglobin, succinic dehydrogenase and other oxidative enzymes, and low myosin ATPase. White fibres, fast twitch and anaerobic, are characterized by the reverse of the above features and have a high glycogen and phosphorylase content. The fibres of a single motor unit (the fibres supplied by the branches of a single neuron) are all of the same type, but if the nerve supply is altered (as by regeneration of nerve fibres after injury) the type becomes altered. In contrast to smooth and cardiac muscle, each individual skeletal muscle fibre receives a motor nerve fibre. Embedded among the ordinary skeletal muscle cells are groups of up to about 10 small specialized muscle fibres that constitute the muscle spindles. The spindle fibres are held together as a group by a connective tissue capsule and hence are called intrafusal fibres (lying within a cigar-shaped or fusiform capsule), in contrast to ordinary skeletal muscles fibres which can be called extrafusal. Muscle spindles are constant in position in any given muscle and are most numerous (relative to the muscle bulk) in muscles concerned with fine movements (e.g. 368 in latissimus dorsi, 80 in abductor pollicis brevis). The intrafusal fibres are innervated by the 7 motor neurons of the anterior horn, in contrast to ordinary {extrafusal) fibres which receive their motor supply from the large a cells. Spindles act as one type of sensory receptor, transmitting to the central nervous system information on the state of contraction of the muscles in which they lie. The afferent fibres (types la and II, p. 17) come from primary (annulospiral) and secondary (flowerspray) endings which wrap themselves round parts of the spindle cells. SKELETAL MUSCLES The disposition of the individual fibres in a muscle can be in one of only two ways, namely, parallel or oblique to the line of pull of the whole muscle. In the former maximum range of mobility is assured, in the latter the range of mobility is less but increased force of pull of the muscle is correspondingly gained. A good example of a muscle with parallel fibres is provided by sartorius. In flexing the knee and hip and laterally rotating the hip the muscle is contracted to its shortest length; reversing the movements elongates the muscle by 30%. Other examples of muscles with parallel fibres are rectus abdominis, the infrahyoid group, the extrinsic eye muscles, the anterior and posterior fibres of deltoid, the flank muscles of the abdomen, and the intercostals. Muscles whose fibres lie oblique to the line of pull of the whole muscle fall into several patterns: (1) Unipennate muscles. The tendon forms along one margin of the muscle and all the fibres slope into one side of the tendon, giving a pattern like a feather split longitudinally. A good example is flexor pollicis longus. (2) Bipennate muscles. The tendon forms centrally, usually as a fibrous septum which enlarges distally to form the tendon proper. Muscle fibres slope into the two sides of the central tendon, like an ordinary feather. An example is rectus femoris (in which muscle the fibres slope upwards towards the central septum). (3) Multipennate muscles. These are of two varieties: a series of bipennate masses lying side by side, as in the acromial fibres of deltoid, subscapularis, etc.; and a cylindrical muscle within which a central tendon forms. Into the central tendon the sloping fibres of the muscle converge from all sides. An example is the tibialis anterior. A further variety, unique, is provided by soleus. The main bulk of this powerful muscle consists of short fibres that slope between two aponeu roses, downwards from the deep to the superficial. In a muscle whose fibres run parallel with its line of pull a given shortening of muscle fibres results in equal shortening of the whole muscle. In unipennate and multipennate muscles a given shortening of muscle fibres results in less shortening of the whole muscle. The loss of shortening is compensated by a corre sponding gain in force of pull. Obliquity of pull of a contracting fibre involves a loss of mechanical efficiency. But the number of oblique fibres is much greater than the number of longitudinal fibres required to fill the volume of a long muscle belly. The greater number of oblique fibres, though each fibre loses some efficiency, results in an overall gain of power in the muscle as a whole. Such muscles are found where great power and less range of movement are needed. Surface appearance Some muscles are wholly fleshy, some largely aponeu rotic, while many have a quite characteristic mixture of the two. Such variations provide an illustration of the relation of form to function. If the surface of a muscle bears heavily on an adjacent structure it will be covered by a glistening aponeurosis; where there is no pressure there is usually flesh. Examples are manifold, and in this book the surface appearances of many muscles are described. Here rectus femoris may be cited as a good example. The anterior surface of this bipennate muscle is fleshy where it lies beneath the fascia lata, being INTRODUCTION TO REGIONAL ANATOMY aponeurotic only at its upper end, where it plays against a fibrofatty pad that separates it from sartorius. Its deep surface is exactly the reverse. At the upper end is flesh, but the remainder of the posterior surface is wholly aponeurotic, where the muscle plays heavily on a corre sponding aponeurosis of the anterior surface of vastus intermedius. The advantage of knowing the surface characteristics of muscles should be obvious to both physician and surgeon. For the physician, for example, the diagnosis of 'rheumatic' pain or tenderness will often hinge on whether the site is over aponeurosis or flesh, for where there is an aponeurosis there is a bursa. Such bursae are often very extensive and are usually open at one end, so that effused fluid never distends them, but a 'dry' inflammation comparable to 'dry' pleurisy will produce pain on movement and tenderness on pressure over these aponeuroses. The surgeon sees muscles far more often than bones, and instant recogni tion of a muscle by its surface appearance gives great confidence and accuracy at operation. Origins and insertions There is no reality in these terms, though the sanctity of long usage forces their continued use. 'Attachment' seems the best alternative for both origin and insertion, and is rightly gaining in popularity. The upper attach ment is usually the origin and the lower attachment the insertion, but sometimes the lower end is considered the origin (rectus abdominis, popliteus). But which end of a muscle remains fixed and which end moves depends on circumstances and varies with most muscles. The insertion of a tendon when, as usually, it is near a joint, is almost always into the epiphysis. An excep tion is the tendon of adductor magnus, the insertion of which into the adductor tubercle is bisected by the epiphyseal line of the femur. Bone markings Fleshy origins generally leave no mark on the bone, though often the area is flattened or depressed and thus visible on the dried bone (e.g. pectoralis major on the clavicle). Contrary to usual teaching, insertions of pure tendon, like the attachments of ligaments, almost always leave a smooth mark on the bone, though the area may be raised into a plateau or depressed into a fossa (spinati, tibialis anterior, patellar ligament, cruciate ligaments on femur, psoas, obturator tendons on femur, etc.). Rough marks are made where there is an admixture of flesh and tendon, or where there is a lengthy insertion of aponeurosis (e.g. ulnar tuberosity, gluteal crest, linea aspera). A characteristic of flat muscles that arise from flat bones and play over their surfaces is that the muscle origin does not extend to the edge of the flat bone. The origin of the muscle is set back from the edge of the bone in a curved line. Between the edge of the bone and the curved line is a bare area, over which the contracting muscle slides. This allows a greater range of movement of the contracting muscle fibres. The bare area is invariably occupied by a bursa, and such bursae are always of large size. The bursa may communicate with the nearby joint (e.g. subscapularis, iliacus) in which case infection of one cavity necessarily involves the other. Some of these bursae remain separate from the nearby joint (e.g. supraspinatus, usually infra spinatus, obturator internus). The temporalis muscle is an exception to this rule, for its fibres arise from the whole of the temporal fossa down to the infratemporal crest, and there is no bursa beneath it. Muscle action Movements are the result of the co-ordinated activity of many muscles, usually assisted or otherwise by gravity. Bringing the attachments of a muscle (origin and inser tion) closer together is what is conventionally described as the 'action' of a muscle (isotonic contraction, short ening it). If this is the desired movement the muscle is said to be acting as a prime mover, as when biceps is required to flex the elbow. A muscle producing the opposite of the desired movement — triceps in this example—is acting as an antagonist; it is relaxing but in a suitably controlled manner to assist the prime mover. Two other classes of action are described: fixators and synergists. Fixators stabilize one attach ment of a muscle so that the other end may move, e.g. muscles holding the scapula steady are acting as fixators when deltoid moves the humerus. Synergists prevent unwanted movement; the long flexors of the fingers pass across the wrist joint before reaching the fingers, and if finger flexion is the required movement, muscles such as flexor and extensor carpi ulnaris act as synergists to stabilize the wrist so that the finger flexors can act on the fingers. A muscle that acts as a prime mover for one activity can of course act as an antagonist, fixator or synergist at other times. Muscles can also contract isometrically, with increase of tension but the length remaining the same. They can be assisted by gravity, and may contract paradoxically, as in 'paying out rope', e.g. when biceps (a flexor) controls extension of the elbow. Many muscles can be seen and felt during contraction, and this is the usual way of assessing their activity, but sometimes more specialized tests such as electrical stimulation and electromyography may be required. 8 LAST'S ANATOMY Blood supply Muscles have a rich blood supply. The arteries and veins usually pierce the surface in company with the motor nerve. From the muscle belly vessels pass to supply the adjoining tendon. Lymphatics run back with the arteries to regional lymph nodes. Nerve supply Skeletal muscle is supplied by somatic nerves (p. 16) through one or more motor branches which (in spinal nerves) also contain afferent and autonomic fibres. The efferent fibres in spinal nerves are the axons of the large a anterior horn cells of the spinal cord which pass to extrafusal fibres, and of the small 7 cells which supply the spindle (intrafusal) fibres (p. 6). The motor nuclei of cranial nerves provide the axons for those skeletal muscles supplied by cranial nerves. The pathways for neuromuscular control are considered on page 623. The flat muscles of the body wall are perforated by cutaneous nerves on their way to the skin. Such nerves do not necessarily supply the muscles. However, in the limbs it is a fact that whenever a nerve pierces a muscle it supplies that muscle, and the motor branch leaves the nerve proximal to the muscle. As a matter of morpholog ical fact, limb nerves do not pierce muscles at all, but pass actually in planes between distinct morphological masses that have fused together. Whenever a nerve pierces a muscle, suspect a morphological pitfall (e.g. coracobrachialis, supinator, sternocleidomastoid). Note that all muscles in a limb are supplied by branches of the limb plexus, and that flexor muscles derive their nerve supply from anterior divisions and extensor muscles from posterior divisions of the nerves of the plexus (p. 21). The nerve to a muscle in the body wall or in a limb contains some 40% of afferent fibres. These innervate muscle spindles and mediate proprioceptive impulses. They are indispensable to properly co-ordinated muscle contraction. The nerves supplying the ocular and facial muscles (third, fourth, sixth and seventh cranial nerves) contain no sensory fibres. Proprioceptive impulses are conveyed from the muscles by local branches of the trigeminal nerve. The spinal part of the accessory nerve and the hypoglossal nerve likewise contains no sensory fibres. Proprioceptive impulses are conveyed from sternoclei domastoid by C2 and 3 and from the trapezius by C3 and 4, and from the tongue muscles probably by the lingual nerve (from the trigeminal). Sensory pathways form an essential background to coordinated voluntary movements. They mediate propri oceptive information from the muscle, its tendon, and the capsule and ligaments of the joint being acted upon. These pathways are stimulated in the clinical investiga tion of tendon reflexes (p. 28). BONE Bone is a type of dense connective tissue with cells and fibres embedded in a calcified ground substance commonly called bone matrix. For a tissue that seems so dense and hard perhaps one of its most surprising features is its vascularity. The cells of hyaline cartilage, and indeed of other connective tissues, obtain their nutrition by diffusion through the surrounding intercel lular substance, but the cells of bone, lying in their calcified matrix through which diffusion is impossible, have been deprived of this facility. A system of minute channels therefore developed in bone matrix so that each osteocyte (bone cell) can still receive nutritive substances. The smallest channels are the bone canali- culi which communicate with larger spaces, the Haversian canals, containing the blood capillaries from which plasma can diffuse. During bone development the matrix is laid down in concentric layers (lamellae) around the capillaries. Volkmann's canals are channels that usually run at right angles to the Haversian canals and contain anastomosing vessels between Haversian capillaries. Macroscopically bone exists in two forms, compact and cancellous. Compact bone is hard and dense, and resembles ivory, for which it is often substituted in the arts. True ivory is dentine. Cancellous bone consists of a spongework of trabeculae, arranged not haphaz ardly but in a very real pattern best adapted to resist the local strains and stresses. If for any reason there is an alteration in the strain to which cancellous bone is subjected there is a rearrangement of the trabeculae. The moulding of bone results from the resorption of existing bone by phagocytic osteoclasts and the deposi tion of new bone by osteoblasts; but it is not known how these activities are controlled and co-ordinated. Although adult bone exists in both cancellous and compact forms, there is no microscopic difference between the two. The marrow cavity in long bones and the interstices of cancellous bone are filled with marrow, red or yellow as the case may be. This marrow apparently has nothing whatever to do with the bone itself, being merely stored there for convenience. At birth all the marrow of all the bones is red, active haemopoiesis going on everywhere. As age advances the red marrow atrophies and is replaced by yellow, fatty marrow, with no power of haemopoiesis. This change begins in the INTRODUCTION TO REGIONAL ANATOMY distal parts of the limbs and gradually progresses proxi mally. By young adult life there is little red marrow remaining in the limb bones, and that only in their cancellous ends; ribs, sternum, vertebrae and skull bones contain red marrow throughout life. Periosteum and endosteum The outer surfaces of bones are covered with a thick layer of fibrous tissue, in the deeper parts of which the blood vessels run. This layer is the periosteum and the nutrition of the underlying bone substance depends on the integrity of its blood vessels. The periosteum is itself osteogenic, its deeper cells differentiating into osteoblasts when required; hence the deeper part is known as the osteogenic layer. In the growing individual new bone is laid down under the periosteum, and even after growth has ceased the periosteum retains the power to produce new bone when it is needed, e.g. in the repair of fractures. The periosteum is united to the underlying bone by Sharpey's fibres, particularly strongly over the attachments of tendons and ligaments. Periosteum does not, of course, cover the articulating surfaces of the bones in synovial joints; it is reflected from the articular margins, to join the capsule of the joint. The single-layered endosteum that lines inner bone surfaces (marrow cavity and vascular canals) is also osteogenic and contributes to new bone formation. The mass of inflammatory tissue and bone-forming cells at a fracture site constitutes the fracture callus, and it is more accurate to refer to osteogenic cells as osteochon- drogenic: in a well-vascularized area they are indeed osteogenic and produce new bone, but in areas that become avascular the cells are chondrogenic and form hyaline cartilage. Excessive cartilage formation instead of bone is characteristic of fracture sites that remain mobile. Nerve supply Subcutaneous periosteum is supplied by the nerves of the overlying skin. In deeper parts the local nerves, usually the motor branches to nearby muscles, provide the supply. Periosteum in all parts of the body is very sensitive. Blood supply In the adult the nutrient artery of the shaft of a long bone usually supplies little more than the bone marrow. The compact bone of the shaft and the cancel lous bone of the ends are supplied by branches from the periosteum, especially numerous beneath muscular and ligamentous attachments. Before union with the shaft an epiphysis is supplied from the circulus vasculosus of the joint (p. 12). Veins are numerous and large in the cancellous red marrow bones (e.g. the basivertebral veins) and run with the arteries in Volkmann's canals in compact bone. Lymphatics are present, but scanty; they drain to the regional lymph nodes of the part. Development Bone develops by two main processes, intramembra- nous and endochondral ossification (ossification in membrane and cartilage). In general the bones of the vault of the skull, the face and the clavicle ossify in membrane, while the long bones of the skeleton ossify in cartilage. In intramembranous ossification, osteoblasts simply lay down bone in fibrous tissue; there is no carti lage precursor. As well as the bones of the skull vault, face and the clavicle, it should be noted that growth in the thickness of other bones (subperiosteal ossification) is also by intramembranous ossification. In endochondral ossification a pre-existing hyaline cartilage model of the bone is gradually destroyed and replaced by bone (Fig. 1.5). Most bones are formed in this way. It is essential to appreciate that the cartilage is not converted into bone; it is destroyed and then replaced by bone. During all the years of growth there is constant remodelling with destruction (by osteoclasts) and replacement (by osteoblasts), whether the original development was intramembranous or endochondral. Similarly endochondral ossification, subperiosteal ossification and remodelling occurs in the callus of fracture sites (see above). The site where bone first forms is the primary centre of ossification, and in long bones is in the middle of the Surface cartilage- Growing cartilage Calcified cartilage New bone and osteoblasts Marrow cavity Fig. 1.5 Endochondral ossification in the epiphysis at die end of a long bone. <2>© SO® <3>e> -?^:.'.'.;©'.<5i.'-ffi.SV-:e3©v 10 LAST'S ANATOMY shaft (diaphysis), the centre first appearing about the eighth week of intrauterine life. The ends of the bone (epiphyses) remain cartilaginous and only acquire ossification centres much later, usually after birth. The dates of commencement of ossification in epiphyses (secondary centres of ossification) and of their fusion with the diaphysis were much loved of the older anatomists (and examiners). Although students are no longer required to memorize long lists of dates, which in any case are rather variable, it is still important in radiographs of the young and adolescent to be able to recognize the sites of epiphyseal lines (where an epiphy seal plate of cartilage remains between epiphysis and diaphysis) in order to distinguish them from fracture lines. Secondary cartilage This is the name given to a special type of cartilage that develops in certain growing membrane bones (head of the mandible and the ends of the clavicle). Both these bones are primarily ossified in membrane. Their artic ular surfaces are covered with fibrocartilage, identical with the intra-articular disc in structure. Between the bundles of fibrous tissue are many cells. At some distance beneath the articular surface the cells divide, enlarge, and come to lie close together in a groundwork of cartilage that contains many fibres. This * secondary cartilage' differs in appearance from hyaline cartilage in that its cells are larger and more tightly packed and the matrix is much more fibrous (Fig. 1.6). The secondary cartilage in the neck of the mandible persists until growth of the mandible is complete. It is rather like the epiphysis at the end of a long bone, but unlike the clavicle it has no secondary centre of ossification. Note that secondary cartilage has nothing to do with the production of secondary cartilaginous joints Disc of fibrocartilage Surface fibrocartilage Secondary cartilage New bone and osteoblasts Marrow cavity Fig. 1.6 Ossification in secondary cartilage, as in the head of the mandible. (see below). It provides a cartilage surface for membrane bones that would otherwise have no carti laginous ends. Sesamoid bones Sesamoid bones (meaning seed-like) are usually associ ated with certain tendons where they glide over an adjacent bone. They may be fibrous, cartilaginous or bony nodules, or a mixture of all three, and their presence is variable. The only constant examples are the patella, which is by far the largest, and the one in each of the two tendons of flexor pollicis brevis in the hand and the foot. Occasional sesamoids in the hand may be found in any of the flexor tendons in front of the metacarpophalangeal joints. In the foot they can occur as in the hand flexor tendons, and other possible sites include the peroneus longus tendon over the cuboid, the tibialis anterior tendon against the medial cuneiform, the tibialis posterior tendon opposite the head of the talus, in any of the tendons at the medial and lateral malleoli, and one in the lateral head of gastrocnemius (where it is known as the fabella, an example not associ ated with tendon). The reasons for the presence of sesamoids are obscure. Sometimes they appear to be concerned in altering the line of pull of a tendon (patella in the quadriceps tendon) or with helping to prevent friction (as in the peroneus longus tendon moving against the cuboid bone). JOINTS Union between bones can be in one of three ways: by fibrous tissue, by cartilage or by synovial joints. Fibrous joints exist between bones or cartilages. The surfaces are simply joined by fibrous tissue (Fig. 1.7A) and movement is negligible. Fibrous joints unite the bones of the vault of the skull at the sutures; these gradually ossify (from within outwards) as the years pass by. A fibrous joint unites the lower ends of tibia and fibula; this does not ossify. Cartilaginous joints are of two varieties, primary and secondary. A primary cartilaginous joint is one where bone and hyaline cartilage meet (Fig. 1.7B). The junction of bone and cartilage in ossifying hyaline carti lage provides an example. Thus all epiphyses are primary cartilaginous joints, as are the junctions of ribs with their own costal cartilages. All primary cartilagi nous joints are quite immobile and are very strong. The adjacent bone may fracture, but the bone-cartilage interface will not separate. A secondary cartilaginous joint (symphysis) is a union between bones whose articular surfaces INTRODUCTION TO REGIONAL ANATOMY 11 Periosteum Perichondrium Hyaline cartilage Fig. 1.7 Fibrous and cartilaginous joints in section. A Fibrous joint. B Primary cartilaginous joint. C Secondary cartilaginous joint. are covered with a thin lamina of hyaline cartilage (Fig. 1.7C). The hyaline laminae are united by fibrocartilage. There is frequently a cavity in the fibrocartilage, but it is never lined with synovial membrane and it contains only tissue fluid. Examples are the pubic symphysis and the joint of the sternal angle (between the manubrium and the body of the sternum). An intervertebral disc is part of a secondary cartilaginous joint, but here the cavity in the fibrocarti lage contains a gel (p. 537). A limited amount of movement is possible in secondary cartilaginous joints, depending on the amount of fibrous tissue within them. In spite of the name 'cartilaginous' they have nothing in common with the 'primary' cartilaginous joint. Fully developed synovial joints, which include all limb joints, are characterized by six features: the bone ends taking part are covered by hyaline cartilage and surrounded by a capsule enclosing a joint cavity, the capsule is reinforced externally or internally or both by ligaments and lined internally by synovial membrane, and the joint is capable of varying degrees of movement. The joint capsule is properly called the capsular ligament; do not forget to include it if being questioned about the ligaments of a joint. In the fetus the epiphy seal line gives attachment to the capsule, but this attachment may later wander on to either the epiphysis or the shaft, causing the adult epiphyseal line to be intracapsular or extracapsular (Fig. 1.8). The synovial membrane lines the capsule and invests all non articulating surfaces within the joint, i.e. it is attached round the articular margin of each bone. Certain cells of the membrane secrete a hyaluronic acid derivative which is responsible for the viscosity of the fluid, whose main function is lubrication. It has the extraordi nary capacity of being able to vary its viscosity, becoming thinner with rapid movement and thicker with slow. However, synovial joints should not be compared with machine bearings which require hydro- dynamic lubrication. Under slow-moving weight bearing, hyaline cartilage on joint surfaces possesses an inherent slipperiness greater than that of a skate on ice. Do not imagine that there are large amounts of synovial fluid in joints; in normal joints the fluid is a mere film. The largest joint of all, the knee, only contains about 0.5 ml, but of course injury or disease may cause large effusions. The extent to which the cartilage-covered bone-ends make contact with one another varies with different positions of the joint. When the surfaces make the maximum possible amount of contact the joint is said to be close-packed (as in the knee joint in full extension); the capsule and its reinforcing ligaments are at their tightest. When the surfaces are less congruent (as in the partly flexed knee), the joint is loose-packed and the capsule looser, at least in part. The varying degrees of Attachment in embryo Fig. 1.8 Migration of joint capsules from die epiphyseal line. The epiphysis of the head of the femur becomes intracapsular, while that of the lower end becomes extracapsular. 12 LAST'S ANATOMY contact enable the sites of maximal stress to vary, so spreading the load. Intra-articular fibrocartilages. Discs or menisci of fibrocartilage are found in certain joints, usually but not always in contact with bones that have developed in membrane. They may be complete or incomplete. They occur characteristically in joints in...