AXIAL SKELETON.] pass between their laminoe. Inter- and supra-vinous liga ments connect -adjacent spinous processes, and in the neck the supra-spinons ligament forms a broad band, the liya- mentum nuckce. 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 vertebra. 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 aboye referred to, the range of movement between any two true vertebra) 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-cartilago interposed between the bodies of the vertebrae, which act like elastic pads or buffers to pre vent 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 spinse muscle, the fibres of which pass longitudin ally 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 laminae and spines of adjacent verte brae, and are known as the semispinales, multifidi, and rotatores spinse muscles. The head is balanced on the summit of the spine, and is maintained in a quiescent position without any appre ciable 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, ANATOMY 835 bones and joints play but a small part. The lumbar Muscles < vertebrae behind, the expanded wings of the iliac bones abdomen 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 tho 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, extend ing from the crest of the ilium to the pubic spine ; the ex ternal 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 ape/neuroses, 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 defaeca- tion and micturition. Bones and joints play a more important part in the formation of the walls of the thoracic than of the abdo minal cavity. Not only are there thoracic vertebrae 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 vertebra;, and the seven upper ribs, through their costal cartilages, articulate with the sternum. The Oosto-vertebral Joints are situated between the head Costal of the rib and the vertebral body ; also, except in the float- j oil 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 Costo-sternal Joints are also diarthrodial (except the first costal cartilage, which is directly united to the prae-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 costo- vertebral and costo-sternal joints are of the utmost import ance in the process of breathing. . Breathing or respiration consists of two acts breathing in, or inspiration, and breath ing 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 occixr in the walls of the chest : the ribs are elevated and rotated, the lower borders of their shafts are everted, 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 move ments 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 postici
