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tremities, one of which is the head or the origin, the other the insertion. The belly is the fleshy part of the muscle, and possesses a deepred characteristic colour; it is the active contractile structure, the source of motor power. The two extremities are called the tendons of the muscle, or sinews; the tendons are bluishwhite in colour, possess no power of contractility, and are merely, as it were, the ropes by which the belly of the muscle is attached to the bone or other structure which is moved by its contraction. The term tendon of origin, applied to one extremity of the muscle, signifies the fixed end of the muscle, that to which it draws during its contraction; as a rule this is the end nearest the trunk, the proximal end. The term tendon of insertion is applied to the end which is moved by the contraction; as a rule this is the end most removed from the trunk, the distal end. Entering the substance of each muscle is at least one artery, which conveys blood for its nutrition; this artery ends in a network of capillary blood-vessels, from which a vein arises and conveys the blood out of the muscle again; another small vessel, called a lymphatic, also arises within the muscle, and conveys the fluid lymph out of the muscle. Each muscle also is penemated by a nerve, through which it is brought into connection with the brain, so as to be subject to the influence of the will. The will is the natural stimulus for exciting muscular action, which action is in many cases so rapid that scarcely an appreciable intcrval of time intervenes be- Wmfldymflumcvml, tween willing and doing the action. P‘ ‘be Well"

The bones form a series of rod-like levers, and, in studying the mode of action of the muscles, the place of insertion of the muscle into the bone—that is to say, the point of application of the power which causes the movement— and its relations to the joint, or fulcrum, or centre of motion, and to the weight or resistance which is to be overcome, have to be kept in view. The relative positions of fulcrum, point of application of power, and resistance, are not the same in all the bony levers. As a rule, the muscles are inserted into bones between the fulcrum and the moveable point of resistance, and nearer the fulcrum than the movable point, as may be seen in the muscles which bend the fore-arm at the elbow-joint. Although from the weightarm of the lever being in these cases much longer than the power-arm, the muscles, as regards the application of the power, act at a disadvantage, yet the movement gains in velocity. Sometimes the muscle is inserted, as is the case in the great muscle which straightens or extends the fore-arm, at one end of the lever, and the fulcrum or joint is placed between it and the movable point. At other times, as in the case of the chief depressor muscle of the lower jaw, whilst the muscle is attached to one end of the lever, the fulcrum is at the opposite end. When a muscle is so placed that its tendon of insertion is perpendicular to the bone to which it is attached, it acts to great advantage; when placed obliquely or nearly parallel, a loss of power occurs. Many muscles at the commencement of contraction lie obliquely to the bones which they move,

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Fm. Ill—Tho rcctus muscle of the thigh; to show the const‘uucnr pans of a muscle. R. the fleshy belly; lo, lcndon of orlgln; ti, tendon of inserlion; 1:, nerve of supply; a, srtery of supply; 0,

but as contraction goes on they become more nearly perpendicular, so that they act with more advantage near the close them at the commencement of contraction. If a muscle passes over only one joint, it acts on that joint only _; but if it passes over two or more joints, it acts on them in succession, beginning with the joint next the point of insertion. A given movement may be performed by the contraction of a single muscle, but as a rule two or more muscles are associated together, and they are not unfroquently so arranged that one muscle initiates the move ment, which is then kept up and completed by the rest. Muscles producing movement in one direction have opposed to them muscles which by their contraction effect the opposite movement; when both groups act simultaneously and with equal force, they antagonise each other, and no motion is produced; when a muscle is paralysed or divided, its antagonistic muscle draws and permanently retains the part to its own side. The rapidity of action of a muscle is proportioned to the length of its fasciculi, its power of contraction to their number.

Each muscle is invested by a sheath formed of connective tissue. In the limbs and in the neck not only has each muscle a sheath, but a strong fibrous membrane envelopes the whole of the muscles, and assists materially in giving form and compactness to the region. This membrane is called generally a fascia or aponeurosis, but special descriptive names are given to it in the different regions—cg, cervical fascia, brachial aponeurosis, fascia lata, or fascia of the thigh. In some localities muscles arise from the fascia, and in others they are inserted into it. The fascia is separated from the skin by a layer of subcutaneous fatty tissue, and in this layer muscles are in some localities developed. In the fat of the inner border of the palm of the hand a small muscle, the palmaris brevis, is found, which is inserted into the skin covering the ball of the little finger; at each side of the neck, also, lies a thin muscle called pIatg/sma mg/Oides, and the muscles on the face and scalp which move the skin of the face and head belong to the same category. These muscles form the group of subcutaneous or dermal muscles which, except in the localities above referred to, are not represented in the human body, but are well known in the bodies of the mammalia generally as the panniculus camosus.

In arranging the muscles for descriptive purposes, either a morphological, a topographical, or a physiological method may be pursued. The morphological arrangement is to be preferred when the object is to compare the muscular system in man with that in different animals, and the basis of the arrangement should be into muscles of the axial, the appendicular, and the axi-appendicular skeletons, and sub-cutaneous muscles; a topographical arrangement is most suitable for the purposes of the practical surgeon ,' a physiological arrangement, when the object is to study the action of the muscles in connection with the movements of the joints. In Plates XV. and XVL, a front and back view of the voluntary muscles of the body is given.

J om'rs AND Musctns or THE AXIAL SKELETON.

The Intervertebral Joints are complex in construction. l'fl

The bodies of the true vertebrae are connected together by an amphiarthrodial joint: the fibre-cartilaginous plate or intervertebral disc is tough and fibrous in its pelipheral part, but soft and pulpy within. (Fig. 15.) Remains of the chorda dorsalis are said to occur in the soft pulp, and sometimes a distinct cavity, lined by a synovial membrane, is found in the centre of the disc, which in the finner whales is expanded into a large central cavity containing many ounces of synovia. A diarthrodial joint connects the superior and inferior articular processes of adjacent vertebrae 011 each side. Elastic yellow ligaments, the Iz'gamenta subjlavd'

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ENCYCLOP£DIA BRITANNICA. NINTH EDITION.

pass between their laminae. Inter- and supra-.rpinoua ligaments connect adjacent spinous processes, and in the neck the supra-spinous ligament forms a broad band, the ligamenium nuchw. In those mammals which possess big heads or heavy horns, this ligament of the back of the neck forms a powerful elastic band for the support of the head. The joints between the atlas and axis, and the atlas and occiput, are specially modified in connection with the movements of the head on the top of the spine. The intervertebral discs are absent, and the range of movement either from before backward, as in nodding the head, or from side to side, as in looking over the shoulder, are more extensive than between any of the other true vertebrae. The head rotates along with the atlas around the odontoid or pivot process of the axis, which is lodged between the anterior part of the atlas and a strong transverse ligament which lies behind the odontoid. Too great movement to one side or the other'is prevented by the check ligaments, which pass from the top of the odontoid to the occipital bone, in front of the foramen magnum. The nodding movements take place between the occiput and atlas, and are permitted by the size and shape of the occipital condyles and hollow upper articular surfaces of the atlas. These joints are all diarthrodial. The spine is flexible and elastic; except in the joints above referred to, the range of movement between any two true vertebrae is very small, but the sum of the movement in the entire spine, owing to the number of bones, is considerable. The elasticity of the spine is partly due to the numerous diarthrodial joints between its articular processes, but more especially to the discs of fibro-cartilage interposed between the bodies of the vertebrae, which act like elastic pads or buffers to prevent shock. The spine and trunk may be bent either forwards or backwards, or to the right and left side; or without being bent, the spine may be screwed to the right or to the left, the screwing movement being permitted by the oblique direction of the articular processes.

The muscles which move the vertebrae on each other are principally situated on the back of the trunk. In the hollow on each side of the vertebral spines lies the great erector spins: muscle, the fibres of which pass longitudinally upwards. When both muscles act together, the entire spine is bent back; but when the muscle of one side only contracts, then the spine is bent to that side. These muscles also act in raising the spine from the bent to the erect position, and they are assisted by small inter-spinal muscles, situated between the spines in the cervical and lumbar regions. The spine is bent forward by the psoae and longi colli muscles; and the straight muscles of the abdomen, inserted into the lower true ribs, assist in this movement. The screwing movements of the spine are effected by a series of muscles, the fibres of which pass obliquely between the lamina: and spines of adjacent vertebrae, and are known as the semispinales, multifidi, and rotatores spinae muscles.

The head is balanced on the summit of the spine, and is maintained in a quiescent position without any appreciable muscular action, but it can be moved in various directions by the muscles inserted into its bones. The nodding movements of the head on the atlas are due to the posterior recti, the two superior obliques, the two splenii, and the two complexus muscles, which draw it backwards, and the anterior recti and sterno-cleido-mastoid muscles, which draw it forwards. When the right splenius and greater posterior rectus and inferior oblique act along with the left complexus and sterno-mastoid, the head is rotated to the right shoulder; the opposite rotation being due to the action of the corresponding muscles on the other side of the body.

In the formation of the walls of the abdomen proper,

bones and joints play but a small part. The lumbar Muscles of vertebrae behind, the expanded wings of the iliac bones flbdomen

below, and the false ribs above, are the only bones to be considered. Three pairs of greatly expanded muscles— the external oblique, internal oblique, and transverse—lie at the sides and in front, and two pairs of muscles the recti and pyramidales—are situated wholly in front. The internal oblique and the transverse muscles are attached above to the ribs, behind to the lumbar spine, below to the iliac crest and to a strong band, Poupart’s ligament, extending from the crest of the ilium to the pubic spine; the external oblique has similar connections above and below, but is not attached behind to the lumbar spine. The muscles all terminate in front in strong expanded tendons, called the anterior abdominal aponeuroscs, which blend together in the middle line anteriorly to form the band called linea alba, which stretches longitudinally from the xiphi- ternum to the pubic symphysis. These expanded tendons enclose the recti muscles, which pass from the pubis upwards to the cartilages of the lower true ribs, and the pyramidal muscles, which pass from the pubis to be inserted into the

hnea alba. The entire arrangement is admirably adapted '

for completing the walls of the great abdominal chamber, and for enabling the muscles to compress the abdominal viscera, an action which takes place when the contents of the bowels and bladder are being expelled during defaecation and micturition.

Bones and joints play a more important part in the formation of the walls of the thoracic than of the abdominal cavity. Not only are there thoracic vertebrm behind, and the sternum in front, but on each side the twelve ribs arch more or less completely forward from the spine; each rib is articulated behind to one or two vertebrae, and the seven upper ribs, through their costal cartilages, articulate with the sternum.

The Costomertebral Joints are situated between the head Qqstal of the rib and the vertebral body ; also, except-in the float- 10111

ing ribs, between the tubercle of the rib and the transverse process of the vertebra, the joints being diarthrodial, and completed in the usual manner by ligaments and synovial membrane. The Costostemal Joints are also diarthrodial (except the first costal cartilage, which is directly united to the prze-sternum), a capsular ligament, lined by a synovial membrane, connecting the cartilages of the true ribs to the sternum. The cartilages from the sixth to the ninth ribs are also united by ligamentous fibres.

The movements of the ribs and sternum at the costovertebral and costo~sternal joints are of the utmost importance in the process of breathing. Breathing or respiration consists of two acts—breathing in, or inspiration,and breathing out, or expiration. During inspiration, the air rushes through the nose or mouth down the windpipe, and dilates the air-cells of the lungs; together with the expansion of the lungs the walls of the chest rise, so that the capacity both of lungs and chest at the end of a full inspiration is nearly doubled. During inspiration the following changes occur in the walls of the chest: the ribs are elevated and rotated, the lower borders of their shafts are cverted, while their surfaces are at the same time rendered more oblique, and the width of the intercostal spaces is thereby increased; the elevation and rotation of the ribs throw the sternum upwards and forwards, and make the thoracic part of the spinal column straighter; the diaphragm is depressed, and the antero-lateral walls of the abdomen are thrown forward. The muscles which cause these movements are as follows: In each of the spaces between the different ribs a pair of intercostal muscles is situated; these elevate and rotate the ribs, and the movements are assisted by the levatores costarum, and, in the case of the upper and lower ribs, by the scaleni and serrati pcstici

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Joints and muscles of the head.

muscles; and by these agents the transverse and anteroposterior diameter of the chest is increased. The increase in its vertical diameter is due to the action of the diaphragm or midrifl', the great muscle which, arising by its circumference from the xiphi-sternum, six lower ribs, and bodies of the lumbar vertebrae, forms the floor of the thoracic and the roof of the abdominal cavity. It constitutes a great arch, with its convexity directed to the cavity of the chest. By the contraction of its fibres the arch is rendered less convex, and the floor of the chest is thereby depressed. Under circumstances which require more powerful efforts of inspiration, the muscles which pass from the walls of the chest to the upper limbs may, by taking their fixed points at the limbs, act as elevators of the ribs. During expiration the ribs are depressed, their lower borders inverted, the width of the intercostal spaces diminished, the sternum depressed, the spine more curved, and the dia

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Flo. 19.—The concave abdominal surface of the diaphragm. a, 4th lumbar vertebra; I». 0, 12m and 11th ribs; d, xiphi-eiernum; sf, crura of diaphragm; 9, h._nrched tendons of origin of diaphragm; k, aorta; l, oesophagus; m, inferior vrnn can; u, psoas; o. qundrui-us muscle: qqq, central tendon of diaphragm, into which the muscular fibres are inserted.

phragm more convex. These movements are principally due to the recoil of the elastic tissue of the lungs previously rendered tense by the inflation of the air-cells, and to the untwisting of the ribs when the inspiratory muscles cease to elevate and rotate them. Muscular action plays but a small part in quiet expiration, but the expulsion of the air from the lungs may be facilitated by contracting the abdominal muscles, which, pressing the abdominal viscera against the under surface of the diaphragm, force that muscle upwards.

The Tempura-maxillary Joints are the only diarthrodial articulations in the head. The condyle of the lower jaw on each side is received into the glcnoid fossa of the temporal bone; each joint is enclosed by a capsular ligament, and between the articular surfaces is a meniscus, which subdivides the interior of the joint into two cavities, each lined by a synovial membrane. The movements of the lower jaw take place simultaneously at both its articulations during mastication and speech, through the action of the several muscles which are inserted into it. This bone is elevated by the temporal muscles, inserted into the coronoid processes; and by the masseterics, inserted into the outer surface, and the internal pterygoids, into the inner surface of each angle. It is depressed partly by its own weight and partly by the action of the digastrics and genio-hyoids, inserted close to the symphysis; by the platysma, inserted into the outer surface of each horizontal ramus; and the mylo

hyoids, into their inner surfaces. The elevators of the jaw are much more powerful than the depressors, for they not only have to overcome the weight of the bone, but during mastication have to exercise force sufficient to cut or break down the food between the teeth. In carnivorous animals, more especially those which, like the tiger or hyaena, crack the bones of their prey, these muscles attain a great size. The lower jaw can be projected in front of the upper by the external pterygoid muscles, inserted into the neck of the bone on each side; but excessive movement forward is checked by the action of the stylo-maxillary ligaments, which pass from the styloid processes to the angles of the bone ; when projected forward, the jaw is drawn back by the posterior fibres of the temporal muscles. \Vheu the elevator, depressor, protractor, and retractor muscles are successively brought into action, the lateral. or grinding movements of the bone, so important in mastication, are produced.

Along with the movements of the lower jaw those of the hyoid bone and larynx must be considered, for the digastrics. the genio- and mylo-hyoids, which depress the lower jaw, act, when their action is reversed, along with the stylohyoid muscles in elevating the hyoid bone and larynx, which structures can be depressed or drawn downwards by the action of the sterno—hyoids, sterno-thyroids, thyrohyoids, and omo-hyoids ; the elevation of the hyoid, when drawn down by its depressor muscles, is effected by the elastic stylo-hyoid ligaments attached to its small cornua, which, by their recoil when the depressor muscles have ceased to contract, draw the bone up to its former position.

Numerous muscles are situated immediately beneath the skin of the scalp and face. They are not of so deep red :1 colour as the muscles of the trunk and limbs, and whilst they arise from one or other of the bones of the head, they are inserted into the deep surface of the skin itself. Hence when they contract they move the skin of the scalp and face, and as they are the instruments through which the various passions and emotions are expressed, they are grouped t0gether as the dluscles of Expression (Plate XV.__ figs. 2 and 3). The occipito-frontalis, or great muscle of the scalp, passes from the occipital bone over the vertex to the forehead , when it contracts, the skin of the forehead is wrinkled transversely, the eyebrows are elevated, and an expression of amazement or surprise is produced. Some persons have a greater power over this muscle than others, and by the alternate contraction of its occipital and frontal portions can move the hairy scalp to and fro with great rapidity. A pair of muscles, the corrugatores supercilii, arises from the supmciliary ridges, on the frontal bone, to be inserted into the eyebrows: they draw the eyebrows downwards and inwards, wrinkle the skin of the forehead longitudinally, and contract with great vigour in the act of frowningThe auricle of the external ear has three small muscles inserted into it, one behind, the posterior, one above, 111,18 superior, one in front, the anterior auricular muscle: in man, as a rule, these muscles are feeble, and have little action ; but in many mammals they are large, and by them the animal pricks its ears to detect the faintest sound of danger. The eyelids are drawn together, so as to close the eye as in the act of sleep, by the orbicularis palpebrarum, the fibres of which lie in the eyelids and on the borders of the orbit, and surround the fissure between the eyelids This muscle is a characteristic specimen of the group 0 sphincter muscles, 'i.e., muscles which surround orifices, ‘m by their contraction close them. When the upper fibres of the muscle alone contract, the uoper eyelid is depressed,” a movement which takes place almost involuntarily ‘"1 with great frequency during our waking hours, so as to wash the tears over the exposed part of the eyeball and keep it moist. In expressing a “ knowing winki” the

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