DENTITION OF MAMMALS. 535 



tures 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 

 ducts of the glands are dilated into large reservoirs, from 

 which the contents are injected into the mouth of the young 

 animal by the action of a compressor muscle." In all other 

 Mammals the young suck the milk from the mammse. 



Dentition. — The teeth of Mammals are developed in 

 the gums or soft membrane covering the borders of the 

 premaxillas, maxillae, and mandibles. As in other animals, 

 they are in part of epidermic, in part of dermic origin. In 

 the course of their development their bases are enclosed in 

 sockets formed in the subjacent bones. 



In most teeth there are three or four different kinds of 

 tissue. The greater part consists of dentine or ivory ; out- 

 side of this there is a layer of very hard glistening enamel; in 

 the interior there is a cavity which in growing teeth contains 

 a gelatinous tissue or pulp, supplied by blood-vessels and by 

 branches of the fifth nerve, and contributing to the increase 

 of the dentine ; lastly, around the narrowed bases or roots 

 of the tooth, or between the folds of the enamel if these 

 have been developed, there is a bone-like tissue called the 

 crusta petrosa or cement. 



The development of a tooth begins with the formation 

 of an enamel-germ, an invagination of the ectodermic 

 epithelium or epidermis. Beneath this germ a papilla of 



