INTRODUCTION. 



parts of the vertebral column. The tail especially 

 exhibits all possible variations in the vertebra: 

 composing it, both with respect to their number 

 and their form. It may be excessively long, or, 

 as ill the anthropoid apes, quite rudimentary. 



Only in the mammals is the abdominal cavity, 

 which lies partly between the ribs, separated off by 

 a muscular partition, the diaphragm, from the cavity 

 of the chest containing the lungs and heart. 



Special Characters: (a) The Skin and its 

 Modifications. — It must further be mentioned as 

 a specially noteworthy fact, that the two exclusive 

 mammalian characters, the hair and the milk- 

 glands, are both developed from the skin, which, 

 as in all vertebrates, is composed of two distinct 

 layers, the outer or scarf skin, and the inner or 

 true skin. 



The scarf-skin or epidermis undergoes the most 

 various modifications. It consists primarily of 

 layers of contiguous cells forming a thin flexible 

 membrane; but it afterwards thickens in certain 

 places, becomes horny and stiff, acquires a further 

 and often very complicated structure, and forms 

 hard callosities, nails, hoofs, horns, and scales. 

 Among these structures belonging to the scarf-.skin 

 the hairs, nails, and glands are the most widely 

 diffused. 



Hair. — In no mammal is the hair altogether 

 wanting, and though in some, such as the whales, 

 it is only very scanty or apparently not present at 

 all, yet traces of it can always be found either in 

 the young animal or in hidden parts in folds of the 

 skin in the adult. Each hair is formed in a depres- 

 sion of the scarf-skin, and it may assume the most 

 diverse forms. We meet with all transitions from 

 the finest woolly or silky down to large coarse 

 bristles and even spines, which serve as weapons of 

 defence, and appear to consist, like the scales of the 

 scaly ant-eaters, of hairs that have grown together. 

 The horns of the rhinoceros, the horny sheaths of 

 animals with hollow horns, as well as nails and 

 hoofs, appear likewise to consist of agglutinated 

 fibres or small plates, often separated from each 

 other by a considerable amount of pulp. 



In most cases the hairy covering of a mammal 

 is made up of two sorts of hair, soft downy hairs, 

 and a stronger kind, which in certain places de- 

 velops into manes, tufts, beards, and so forth. 

 The depressions or follicles in which the hairs are 

 set are always richly supplied with nerve-endings, 

 which convey impressions of touch. The tactile 

 sensibility conferred thereby is peculiarly well de- 



veloped in the often very long and thick hairs 

 which form a moustache in many mammals. The 

 general character of the hair-covering, which is in 

 many cases splendidly coloured, forms an impor- 

 tant point in zoology. 



Equal importance is attached to the nails, claws, 

 and hoofs, in short to all the horny structures, 

 which are almost always present on the toes of 

 mammals, being wanting only in certain types 

 living in the water, such as the whales. 



Nails. — The nails, which in the apes and mon- 

 keys are flat, and in beasts of prey and many other 

 mammals curved and hook-like, cover only the 

 back of the last phalanx of the toes, while the 

 hoofs envelope it completely. This difference has 

 been employed to distinguish the hoofed mammals 

 {Ungulata) from the nailed mammals (Ungiticn- 

 lata); but this division, in consequence of the large 

 number of intermediate forms, has nothing like the 

 value which was at first attributed to it. 



Superficial Glands Superficial glands, origi- 

 nally formed by an involution of the scarf-skin, are 

 almost universal. They are absent only in some 

 whales. Some of these, the sebaceous glands, 

 secrete an oily slime and are intimately connected 

 with the hair-covering; others, the sweat-glands, 

 appear rather to .serve for the excretion of liquids 

 and gases. Those of the first kind may, in certain 

 situations, grow to a very considerable size. This 

 happens mostly in the neighbourhood of the anus 

 and the reproductive organs, but sometimes also on 

 the head, neck, back, feet, and other parts. In gen- 

 eral such large glands are connected with the repro- 

 ductive functions. The products excreted by them 

 are often of a disagreeable penetrating odour, and 

 these glands may therefore serve even as weapons 

 of defence, as in the polecats and .skunks. Glands 

 of this nature yield musk, the castoreum of the 

 beaver, and similar products. 



Milk-glands. — The most important of the skin- 

 glands finally are those which we call milk-glands. 

 They are never absent, but they act only in the 

 female after the birth of the young, and their 

 secretion, milk, which contains all the ingredients 

 necessary for the nourishment of the body, serves 

 to feed the new-born offspring for a longer or shorter 

 period. Like most of the skin-glands they are 

 made up of longer or shorter tubes opening into 

 each other at certain places, and finally opening to 

 the exterior by very delicate canals. It is only in 

 the monotremes that they are seen in their original 

 form, namely, as a bundle of tubes opening separately 



