540 CHORDATA 



ami cortical layers. On the outsitle there may he another layer recalling 

 the pseiulocuticula of reptiles. In most mammals there is a periodic 

 sheikhng and renewal of the hair, the new hair arising 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 di\'ided 

 into claws [iingiics), hoofs [tiugulcc), and nails {Uidiihc). See p. 450. 



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

 has recently found both defenders and opponents, the latter thinking it probable 

 that the hair has arisen from the neuromasts of aquatic \ertebrates, which in 

 the transition to a terrestrial life, became functionless and cornificd. Where 

 scales are present, the hair is regularly grouped around them. The same sort of 

 grouping occurs in the scaleless mammals, often only in the embryo, a fact 

 that leads to the Wew that the scales are an inheritance from the ancestral 

 mammal, which possessed them in addition to the hair, a view that is opposed 

 to any homology between hair and scale. 



The skin of mammals is further characterized by its richness in glands, 

 of which there are two kinds, sebaccaus and sural glands. The first are 

 acinous, and usually open in the hair follicles, giving the hair the required 

 oiliness (fig. 596, 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, wliich is of great value in the 

 preservation of a constant temperature, its evaporation cooling the body. 

 Under the influence of sexuality the glands in certain regions, and especi- 

 ally the sebaceous glands, develop great activity and form considerable 

 glandular pouches or pockets: caudal and anal glands of many carru^•ores, 

 hoof and suborbital glands of ruminants, musk and castor glands of musk 

 deer and beaver (iig. 603, a). More important than these are the modi- 

 fications of dermal glands into mammary or milk glands, which indeed are 

 so characteristic that they have given rise to the name mammalia. In 

 the monotremes the milk glands are very large branched sweat glands; and 

 the same is true for the other mammals, in which they were formeriy 

 regarded as modified sebaceous glands, on account of the somewhat 

 vesicular entls. In Oniilhorhynchns the glands open on a circumscribed 

 area of the skin, wMch in Ecliidua is sharply set off from the surroundings 

 (areola viamnuc). In the higher mammals this area may be elevated 

 directly to a papilla, the true nipple (most mammals, fig. 597, A), or 

 the skin around the gland openings may be elevated in a tubular form 

 as in the teats of the cow {B). The mamm.T are always symmetrically 

 arranged upon the \-entral surface, sometimes in the breast region, but 

 more frequently in the inguinal region. There are at least two, usually 

 more (22 in Centclcs). In general the number corresponds to the maxi- 



