The cavity of the thorax, which is bounded superiorly by the dorsal vertebrie, later ally by the ribs, and inferiorly by the sternum, is completvly separated from tlie abdomi nal cavity in mammals (but in no other vertebrates) by the muscular septum known as the diaphragm, or midriff.
The scapular arch in mammals is comparatively imperfect, its coracoid elernent (see ConAcom BoNts) not being sufficiently developed, except in the monotremata, to reach the sternum, or to meet its fellow in the mesian line. Where the scapula ha,s any bony connection with the sternum, it is through the clavicle or collar-bone, which is frequently absent. The pelvic arch is always composed of the ilium, ischium, and pubis on either aide, and these bones generally coalesce together, as in man, at an early period of life; but in the moaotrentata they remain separate. In the implacental mammals (the mono tremata and marsupialia), the pelvis presents this striking. peculiarity, viz., that from the symphysis (or mesial line of union) two additional bones, termed the marsupial bones, project forwards and outwards, one of whose functions is to support the mar supium, or pouch, which is characteristic of the female marsupials. In the bat, the pelvis is greatly elongated, and the holies do not unite in the mesial line to form a. symphysis, so that the lower part remains open, as in birds; while itt the cetacea, which have no posterior limbs to be supported by the pelvis, that organ is extremely rudimen tary or even entirely absent. As a general rule, the pelvis of mammals is never so broad as in the human subject, and its lateml walls are always relatively smaller, flatter, and longer.
The anterior extremities are always present, although their modes of conformation are very varied, according to the purposes for which they are designed; and the posterior extremities, which are also always present, except in the cetacea, closely resemble the anterior; the difference being greater in man than in any other case, in consequence of the special adaptation of the pelvic extremities for the support of his body in an erect position. The ordinary modifications of these organs are described in the articles HAND and FOOT.—See Owen On the Nature of Limbs.
The teeth of mammals constitute so characteristic a feature in their organization, and are of so much service in their classification, as to require a special notice. The only animals of this clac‘s in which teeth are altogether absent are the true ant-eaters and tho rnonotremata. The number of teeth is generally much more restricted than in reptiles
or fishes. In most mammals it is the same as in man—viz., 32; but the typical number, according to Owen, is 44. The largest number of teeth occurs in the armadillos (in one species of which are 98 simple molars), and in the dolphins, which have from 100 to 190 teeth. 'When the teeth are in these excessive numbers, they are small, nearly equal, and usually of a simple conical form, but excepting in these cases, most mammals have particular teeth for special purposes; thus, the front teeth, front being commonly adapted to effect the first coarse division of the food, are called cutters, or zneisors; and the back teeth, which complete its comminution, grinders, or molars; while the large conical-pointed teeth (of which there is never more than one in each half jaw), which are specially adapted for holding the food while the animal tears it asunder, are called holders, laniaries, or more commonly canines, from being well developed in the dog. The incisors and canines may be absent, but except in the cases previously mentioned, the molars are always present. The mode in which the teeth are implanted in the jaw is characteristic of the class. Excepting in those teeth which grow from persistent pulps (as the front teeth of the rodents, for example), the dental cavity- is closed in at its extremity, and the tooth is prolonged into a fang, which is implanted in a. socket lined by periosteum, to which the exterior of the fang is firmly adherent; there being never a continuous ossification or anchylosis of the tooth to the jaw. Again, the fang of the molars is usually divided into two, three, or even four divergent processes, and there is no known fish or reptile in which even a bifid fang occurs. Teeth are confined in this class to the maxillary, pre-maxillary, and lower maxillary bones, and form only a single row in each; and, in general, teeth are situated in all these bones. In all existing mammals, except man, there is a vacant space between the incisor and canine teeth. INio mammal has more than two sets of teeth; most, however, have two; the first, which are called temporary, deciduous, or milk teeth, being displaced, and succeeded by the permanent teeth. For a description of the structure and principal forms of these organs, we must refer to the article TEETH, and to prof. Owen's magnificent Odontography.