SKIN 721 



for it consists of bony plates developed in the dermis, 

 overlaid by epidermic scales. The median solid horns of 

 the rhinoceros are epidermic outgrowths, comparable to 

 exaggerated warts ; the paired horns of the Ruminants con- 

 sist of epidermic sheaths covering outgrowths of the frontal 

 bones, but extending far beyond these ; the antlers of 

 stags are outgrowths of the frontal bones, are cast and re- 

 grown each year, and are possessed by the males only, 

 except in the reindeer. 



The skin of Mammals, unlike that of Birds, is rich in 

 glands. Sebaceous glands are always associated with the 

 hair follicles, and sudorific or sweat glands are scattered 

 over the skin. 



Specialised glands are also very common, especially those which 

 secrete some strongly odoriferous stuff, scenting which the animals 

 recognise their fellows, their mates, or their young. Often they are most 

 developed in the males, and their activity increases at the pairing 

 season. 



Among the numerous special glands may be noted those which are 

 connected with a perforated spur on the hind-legs of male Monotremes, 

 the sub-orbital glands of antelopes and deer, the anal glands of 

 carnivores, the perineal glands of the civet, the preputial glands of the 

 musk-deer and beaver, the inter-digital glands of the sheep. 



Most characteristic, however, are the mammary glands, 

 functional in female Mammals after parturition. They 

 seem to be specialisations of sebaceous glands, except in 

 Monotremes, in which they are nearer the sudorific type. 

 They consist of branching tubes opening by one or several 

 apertures on the skin. 



In Monotremes the simple glands, compressed by 

 muscles, open by many pores on a bare patch of skin. 

 This is depressed into a slight cup, from which the young 

 lick the milk. In Marsupials the glands open by teats or 

 mammae, generally hidden within a marsupium ; and again 

 the action of surrounding muscles forces the milk into the 

 mouths of the young, which do not seem to be able to suck. 

 An anterior prolongation of the larynx to meet the posterior 

 nares establishes a complete air passage, and enables the 

 young to continue breathing while they are being fed. In 

 Cetacea the milk ducts are dilated into large reservoirs, the 

 contents of which can be rapidly injected into the mouth of 



