424 



CHAPTEE XVIII. 



MAMMALIA. 



The Mammals are all Tvarm- blooded animals, have their skin 

 more or less covered with hair at some period of their life, 

 and produce their young alive (except Monotremes). The 

 young are always nourished for some time after birth by a 

 specially prepared fluid — mUk — which is secreted by the milk- 

 glands or mammae. These glands vary in number in different 

 mammals, and are situated on the thorax or abdomen, or on 

 both regions at the same time. The embryo mammal is always 

 enveloped in an amnion and allantois (fig. 203, am, al). The 

 latter may disappear, or it may remain, partly forming the 

 placenta, a vascular structure that connects the fojtus with the 

 parent. Contrary to what we saw in birds, the skull of a 

 mammal (vide Anatomy of the Horse) articulates with the 

 vertebral column by two condyles, attached to the occipital 

 bone. Other features in the mammalian skeleton by which we 

 can isolate them from the Sauropsida are, first, that the lower 

 jaw articulates with the skull by the squamosal bone direct, 

 and is never united to the quadrate bone. Secondly, the lower 

 jaw consists of two halves or rami, and each ramus of a single 

 bone only, not several as in Birds and Reptiles. A complete 

 separation exists between the thoracic cavity and the abdomen, 

 the dividing membrane or diaphragm being in the form of a 

 thin musculo-tendinous partition. The heart consists of four 

 chambers, the right and left sides being separate. The red 



