CHARACTERS. MAMMALIA. GENERAL CHARACTKRS. 



of the human subject, and the enormously long one of 

 the giraffe, each contain seven vertebrae, although the 

 one constitutes only one-seventh and the other three- 

 sevenths of the entire vertebral column. In the whales 

 the vertebrae of this region of the body are completely 

 united together, to form a single bone. Except in the 

 sloths, all the cervical vertebrae are destitute of ribs, 

 and the spinous processes gradually increase in height 

 as we recede from the head. The first two vertebrae, 

 however, in the Mammalia, present peculiarities of 

 structure which have obtained them distinct names in 

 all systems of anatomy. The first, called the atlas, 

 forms a bony ring, bearing on its upper surface a pair 

 of cuplike depressions for the reception of the promi- 

 nent condyles or articulating tubercles of the base of 

 the skull (see p. 3) ; by means of this articulation the 

 head is enabled to move up and down. The second 

 vertebra is called the axis, from its possessing a peculiar 

 process which projects forward into the ring of the first, 

 and articulates with a flat surface on the inside of its 

 anterior part. By this arrangement the rotatory move- 

 ment of the head is effected. 



The dorsal vertebrae are usually thirteen in number ; 

 but this general rule is liable to many exceptions. The 

 foremost dorsal vertebrae usually have their upper 

 spinous processes greatly developed, especially in ani- 

 mals possessing long necks or heavy heads ; these 

 processes and those of the posterior cervical vertebrae 

 give attachment to a strong ligament (the nuchal liga- 

 ment), which powerfully aids in supporting the head, 

 and in some animals is continued backward as far as 

 the loins. The dorsal vertebrae are distinguished from 

 the rest by their bearing the articulating surfaces for 

 the ribs, which are confined to this region of the body. 

 The ribs are long, usually slender, curved bones, which 

 articulate by their heads with the bodies of two verte- 

 brae, and are nearly always supported by a tubercle 

 against the transverse processes of the hinder of these. 

 The anterior or true ribs are united by cartilaginous 

 pieces with the sternum or breast-bone, which occupies 

 the centre of the anterior or lower part of the chest. 

 Behind these are some shorter ribs, commonly known 

 as false or floating ribs, which are never united directly 

 with the sternum, but only by the intermediation of a 

 common cartilaginous band. 



Of the lumbar vertebrae there are usually six or 

 seven, but the number varies from two to nine. They 

 are usually larger in the body than the dorsal vertebrae, 

 and the lateral processes are often greatly developed ; 

 they are distinguished from the dorsal vertebrae by the 

 absence of ribs, and of the surfaces for the attachment 

 of the latter. Behind the lumbar region comes the 

 sacrum, a single bony piece, which sometimes con- 

 sists of only one vertebra, but is usually composed 

 of three or four amalgamated together, bearing traces 

 of its compound nature in the apertures which indicate 

 the original points of separation of the distinct vertebrae. 

 This bone gives a firm attachment to the pelvis, or 

 supporting arch of the hinder limbs,, which will be 

 described in treating of those members. The caudal 

 vertebrae are usually numerous, amounting to as many 

 as forty-six in the long-tailed mariis. The smallest 

 number of distinct joints is four ; but in the human 



species, and in some others, the caudal region of the 

 vertebral column is reduced to a mere rudiment. 



The structure of the limbs is nearly identical with 

 the description of the typical conformation of the 

 extremities of the vertebrata already given. The 

 anterior limbs are always present in mammals ; the 

 posterior are sometimes deficient. The former are 

 articulated to a shoulder-blade or scapula, Q, a flat 

 and somewhat triangular bone, usually provided with 

 a strong ridge on its upper surface, which lies amongst 

 the muscles upon the anterior ribs. The shoulder- 

 blades are frequently supported in their position by 

 collar-bones or clavicles, which spring from the fore 

 part of the sternum, and at the opposite extremity 

 articulate with the lower part of the shoulder-blade. 

 These, however, are sometimes wanting, or imperfectly 

 developed. The coracoid bones, which form an im- 

 portant part of the supporting arch of the anterior 

 members in Birds and Reptiles, constituting, in fact, a 

 second and even more powerful pair of collar-bones, 

 only occurs in its full development in one small group 

 of mammals ; in the rest it is reduced to a rudimentary 

 condition and amalgamated with the shoulder-blade, 

 of which it forms a small process. 



The anterior limb itself usually consists, as previously 

 stated, of the arm-bone or humerus, R ; the radius and 

 ulna, s, T ; the carpus or wrist, u ; the metacarpus or 

 hand, v ; and the fingers, w. These parts all undergo 

 great modifications, not only as regards their form and 

 comparative size, but also by the amalgamation, or 

 total suppression of some of their subordinate consti- 

 tuents. Thus, in the monkeys, Plate 34, fig. Ill, we 

 generally find all the parts fully developed, and almost 

 equal in perfection to the same parts in man ; in the 

 carnivorous beasts, Plate 33, fig. 105, the various por- 

 tions of the apparatus are still very distinct, but the 

 great mobility they possess in man and the monkeys is 

 already considerably diminished, to adapt the limbs to 

 the purposes of terrestrial progression ; in the seals, 

 Plate 34, fig. 114, and the cetacea, Plate 34, fig. 109, we 

 still recognize the same parts, but with their mutual 

 powers of motion still further limited, to fit them to act 

 as paddles in the water. The ant-eater and the sloth, 

 Plate 33, fig. 107, and Plate 34, fig. 112, also exhibit the 

 same structure, modified in its details to suit particular 

 purposes, and in the latter case displaying a diminution 

 in the number of fingers. With the exception of the 

 aquatic seal and dugong, all the animals to which we 

 have hitherto referred are either terrestrial or arboreal 

 in their habits; but in the bats, Plate 34, fig. 110, we 

 find the anterior limbs adapted for the purpose of flight. 

 In these the arm-bone, R, is not very disproportionately 

 elongated, but the bones of the fore-arm, s, the meta- 

 carpal bones, v, and the phalanges or finger-bones, w. 

 are of immense length, and these, by stretching a j 

 leathery membrane which unites them, enable the bats 

 to raise themselves into the air, and to fly through that 

 element with great swiftness. 



In the terrestrial animals to which we have already 

 referred, the radius and ulna were still capable of a 

 certain amount of rotatory motion, although not to the 

 extent presented by the monkeys. In the herbivorous 

 terrestrial mammals, the toes are terminated by hoofs, 



