THE MAMMALIA. 



at a certain part of the abdominal surface without 

 forming any elevation. In all other mammals 

 there are formed mammary glands provided with 

 warty or even tongue-shaped teats or mamma;, 

 which the young animal can take into its mouth 

 in sucking. These teats may be situated at different 

 parts of the abdominal surface in front of the chest, 

 in the middle of the abdomen, or even far back in 

 the folds of the groin ; but they are always arranged 

 symmetrically in pairs, and their number is related 

 to the number of young born at a birth. While 

 these teats in the whales are very short and hidden 

 in folds of the skin, they attain a very considerable 

 length in some marsupials, among which animals 

 they either lie in a well-formed pouch or at least 

 between two folds of the skin. In the teats are 

 united the secretory ducts of a number of gland- 

 tubes, and there they form a sort of reservoirs 

 opening to the exterior by one or several mouths. 



(b) The Skeleton. We should exceed the limits 

 assigned to our work if we were to go into the 

 details of the structure of the bony skeleton which 

 holds the parts of the body together. We can 

 only touch on certain points which cannot be passed 

 over in a zoological review of the group. 



The Skull. The skull forms, with the exception 

 of the lower jaw, a single piece composed of bones 

 immovably fastened together. Two separate parts 

 can be distinguished, the brain-case and the facial 

 region, the latter including the nose, the eyes in 

 part, and the commencement of the alimentary 

 canal, the mouth. To this firmly built skull with 

 immovable parts is articulated the lower jaw, which 

 is composed of two halves united at the chin, and 

 sometimes, as in the apes and monkeys, completely 

 fused together, sometimes separated by a distinct 

 suture, or connected only by ligaments. The im- 

 movable attachment of the bones of the upper jaw 

 and other bones of the skull to each other and to 

 the brain-case is not found in most other verte- 

 brates, in which it is rather the rule for the bones of 

 the facial region to be separate and movable. The 

 upper jaw always consists of two bones on each 

 side; the inter- or pre-maxillary bones in the 

 middle of the mouth, and the maxilla; forming the 

 sides. But these are not movable on each other, 

 but are connected by sutures, and often become 

 quite fused together at a more or less advanced 

 age. In all vertebrates, except the mammals, the 

 lower jaw is composed of several bones connected 

 by sutures. Only in this class does each half of 

 the lower jaw consist of but one bone. 



We shall have occasion later on to speak of the 

 teeth, which are found only in the bones of the 

 jaws, and form in the upper as in the under jaw 

 only a single row, 1 while in most other vertebrates 

 the teeth may be set in all the bones which go to 

 form the cavity of the mouth. 



The relations between the brain-case and the 

 facial region of the skull vary considerably. The 

 former part is more fully developed the higher the 

 development of the brain, and with it the mental 

 qualities of the animal. The low types, such as the 

 marsupials, have very tiny brains and insignificant 

 brain-cases in comparison with their powerful jaws. 

 But many subsidiary structures may conceal these 

 marked contrasts, especially in the living animal. 

 The cavities known as the frontal sinuses, which are 

 continuations of the cavity of the nose, and are 

 found between the two plates of the frontal bone, 

 may be developed to such an extent that they take 

 up, as in the elephant, considerably greater space 

 than the brain itself. Longitudinal and transverse 

 ridges, which are required for the attachment of 

 the powerful muscles of mastication, may likewise 

 impart to the brain-case a more imposing appear- 

 ance than it would otherwise have. The length of 

 the jaws, which stands in relation to the action of 

 these parts as levers, may undergo considerable 

 variations in accordance with the greater or less 

 degree of savagencss in the instincts of the animal. 

 Yet in spite of these special circumstances affecting 

 the external appearance of the brain-case, it may 

 confidently be asserted that animals have a larger 

 or smaller brain according as they are distinguished 

 by high or low mental endowments, and that the 

 brain-case formed more or less strictly in accordance 

 with the size and shape of the brain enables us to 

 judge approximately of the development of the 

 organ inclosed within it. 



The Limbs. We do not intend now to con- 

 sider in detail the structure of the vertebral column, 

 the ribs and breast-bone, but we must make a closer 

 examination of the limbs, the structure of which 

 has often afforded the fundamental distinctions for 

 the subdivision of the mammals. 



All mammals have originally had paired fore- 

 and hind-limbs, more or less closely connected 

 with the skeleton, in front by means of the bones 

 of the breast and the shoulder-girdle, behind by 



1 There is only one partial exception to this statement, namely, 

 in the sub-division of the rodents known as the Duplicidentata 

 (including the hare and rabbit), in which, as stated in the proper 

 place in the body of the book, there are two small incisors behind 

 the two large incisors in the upper jaw. TR. 



