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 
mamme, 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 
