836 muscles ; and by these agents the transverse and antcro- posterior diameter of the chest is increased. The increase in its vertical diameter is due to the action of the dia phragm or midriff, 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 in verted, the width of the intercostal spaces diminished, the sternum depressed, the spine more curved, and the dia- Fio. 19. The concave abdominal surface of the diaphiagm. <r, 4th lumbar vertebra ; 6, c, 12th and llth ribs ; rf, xiphi-sternum ; e, f. crura of diaphragm ; g, h, arched tendons of origin of diaphragm ; k, aorta ; I, oesophagus ; m, inferior vena cava ; n, psoas ; o, quadratus 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 Temporo-maxillary Joints are the only diarthrodial articulations in the head. The condyle of the lower jaw on each side is received into the gleuoid fossa of the temporal bone ; each joint is enclosed by a capsular ligament, and between the articular surfaces is a meniscus, which sub divides 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 Avhich 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, in serted close to the symphysis ; by the platysma, inserted into the outer surface of each horizontal minus; and the mylo- [MUSCLES OF 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. When tht 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 stylo- hyoid 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, thyro- hyoids, 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 a 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 to gether as the Muscles 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 fore head ; 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 alter nate 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 supra- ciliary ridges, on the frontal bone, to be inserted into the eyebrows : they draw the eyebrows downwards and in wards, wrinkle the skin of the forehead longitudinally, and contract with great vigour in the act of frowning. The auricle of the external ear has three small muscles inserted into it, one behind, the posterior, one above, the 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 surroand the fissure between the eyelids. This muscle is a characteristic specimen of the group of spldnder muscles, i.e., muscles which surround orifices, and 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 and with great frequency during our waking hours, so as to wash the tears over the exposed part of the eyeball and
keep it uioist. In expressing a " knowing wink," tho
