642 MAMMALIA. 



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, and, except in the 

 reindeer, are cast and regrown each year, and possessed by 

 the males only. 



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 foes, or their prey. 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 con- 

 nected 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. From the white blood corpuscles of 

 the abundant vascular supply, and from a degeneration of 

 the cells lining the glandular tubes, the milk is produced. It 

 begins to be produced when the young are born, when, in 

 Placental Mammals, the demand upon the mother through 

 the placenta has ceased. 



In Monotremes, the simple glands, compressed by 

 muscles, open by several 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 the Cetacea, where the prolonged action of sucking 

 would be incompatible with their subaqueous life, the 



