618 CHORDATA. 



Since side branches are lacking, the structure of the hair is more 

 simple than that of feathers, and the forms fewer. Wool is char- 

 acterized by its spiral turns; then there is straight hair which, by 

 increase in size, forms the * whiskers ' (vibrissse) on the upper lip 

 of many mammals, bristles (swine), and lastly the spines of hedge- 

 hogs and porcupines. In the pelts of many animals two kinds of 

 hair may occur, wool below and straight hair outside. Histolog- 

 ically hair consists of cornified cells, often arranged in medullary 

 and cortical layers. On the outside there may be another layer 

 recalling the pseudocuticula of reptiles. In most mammals there 

 is a periodic shedding and renewal of the hair, the new hair aris- 

 ing from the old follicle (? from the old papilla). Ordinarily this 

 occurs only in spring. Besides hair some mammals have true 

 scales. Constant horny structures are the armatures of the tips of 

 the digits, which, according to form, are divided into claws (ungues), 

 hoofs (ungulae), and nails (lamnae). 



The old view that the hair, like feathers, corresponds to the scales of 

 reptiles has recently found both defenders and opponents, the latter think- 

 ing it probable that the hair has arisen from the nerve-end structures of 

 aquatic vertebrates. The claws, together with those of reptiles and birds, 

 must have come from horny scales, which indeed occur in many amphibia 

 as hollow cones capping the toes. The dorsal part of this scale, the claw 

 plate, becomes especially strong, its formation taking place at the base, 

 the root, from whence it is forced forward over the bed (in man the limit 

 of nail formation is shown by the lunule). The ventral part of the scale, 

 the subungua or solenhorn, is poorly developed in true claws because its 

 region is restricted by the arching of the claw plate in both directions, but 

 is more evident in hoofs, in which the plate is curved only horizontally. 

 In the horse it forms the * sole,' lying between the frog and the hoof. It is 

 rudimentary or entirely lost in the nails of apes and man. 



The skin of mammals is further characterized by its richness 

 in glands, of which, with few exceptions, there are two kinds, 

 sebaceous and sweat glands. The first are acinose glands, and usu- 

 ally open in the hair follicles, giving the hair the required oiliness 

 (fig. 645, D}. The sweat glands, except in the monotremes, are 

 entirely independent of the hairs, and are simple tubes, coiled 

 at their deeper ends (SD), secreting a fluid sweat which is of great 

 value in the preservation of a constant temperature, its evapora- 

 tion cooling the body. Under the influence of sexuality the glands 

 in certain regions, and especially the sebaceous glands, develop 

 great activity and form considerable glandular pouches or pockets : 

 caudal and anal glands of many carnivores, hoof glands and sub- 

 orbital glands of ruminants, musk and castor glands of musk deer 



