INTEGUMENT 31 



of the face are usually much longer and stronger than the 

 others, and are provided with striped muscle-fibres. They are the 

 first to appear in the embryo, and the last to be retained in those 

 forms which have lost their hairy covering in connection with an 

 aquatic life (e.g. Cetacea). Between the outer and inner layers of 

 their follicles are blood-spaces and cavernous tissue, and they are 

 well supplied with branches of the trigeminal nerve. The 

 ordinary hairs are also well innervated, especially in the case of 

 nocturnal animals, and are sensory as well as protective in 

 function. Other modifications of the hairs are seen, e.g. in the 

 eye-lashes, the long tail-hairs of most Ungulates, and various other 

 forms of bristles : spines, such as are characteristic of the Hedge- 

 hog and Porcupine, are merely especially strongly developed 

 bristles. 



Hairs, like feathers, are arranged in definite tracts (flumina 

 pilorum), and the fur often consists of finer and coarser elements. 

 A richer hairy covering (lanugo) is often met with in the 

 embryonic condition than in the adult (e.g. in the human foetus) ; 

 and this fact, together with the occasional appearance of abnorm- 

 ally hairy individuals, indicates that at one time Man was dis- 

 tinguished by a far more abundant clothing of hair than at the 

 present day. 



Hairs are most scanty in the Cetacea and Sirenia, in the 

 former of which they are often limited to a few bristles (sinus- 

 hairs) in the region of the lips (Toothed Whales) or chin (Whale- 

 bone-Whales), or may be entirely wanting except in embryonic 

 stages. In the Sirenia, apart from the persistent hairs, a thick 

 coat of fine hairs is present in the embryo, and modified traces of 

 these can be recognised in the epiderm of the adult. 



The hairy coat may be shed and renewed periodically (e.g. in 

 the case of Mammals exhibiting differences in their summer and 

 winter fur), or the shedding and renewal may take place con- 

 stantly, and so result in no marked change of coat. 



Epidermic scales may also occur in Mammals, but are rarely 

 present on parts which are well covered with hair. They are large 

 and well marked in Manis, covering the dorsal surface of the head 

 and body, the sides of the latter and the whole tail, and are 

 present on the tail of various Rodents (e.g. Beaver, Anomalurus, 

 MuridaB), Insectivores, Anteaters, 1 and Marsupials. Other epidermic 

 structures formed as thickenings of the horny layer also play an 

 important part in Mammals : such are, claws, nails, hoofs, the horn- 

 sheaths of Ruminants, the so-called whalebone (baleen) of the 

 Mystacoceti, the palatal plates of Sirenia, the thickened regions of 

 the epiderm in Cetaceans and Pachyderms, the ischial callosities of 

 certain Apes, and the nasal horns of the Rhinoceros, the last- 

 mentioned of which consist of numerous hair-like horny fibres. 



According to the form taken by the horny covering of the 

 1 Vestiges of horny scales also occur in Annadilloes. 



