MAMMALIA. 



CHAPTER XXXI, 



CLASS v.— MAMMALIA. 



The Mammalia include all the ordinary quadrupeds, and 

 may be shortly defined as comprising Vertebrate Animals in 

 which some part or other of the skin is always provided with 

 hairs, and the young are nourished for a longer or shorter 

 time by means of a special fluid — the milk — secreted iy special 

 glands — the mammary glands. These two peculiarities are 

 of themselves sufficient to separate the Mammals from all 

 other classes of the Vertebrate sub-kingdom. In addition, 

 however, to these two leading characteristics, the following 

 points are of scarcely less importance : 



1. The skull is united with the spinal column by means of 

 two articulating surfaces or condyles, instead of one, as in the 

 Reptiles and Birds. 



3. The lower jaw consists of two halves, each composed 

 of a single piece, and united in front. The lower jaw, also, is 

 always jointed directly with the skull, and there is no quad- 

 rate bone. 



3. The heart consists — as in Birds — of four distinct cham- 

 bers, two auricles and two ventricles. The right and left sides 

 of the heart are completely separated from one another, and 

 there is never any direct communication between the blood 

 sent to the lungs and that sent to the body. The red corpuscles 

 of the blood (Fig. 99, a) are, in the great majority of cases, 

 in the form of circular disks, and they never contain any inter- 

 nal solid particle or nucleus. 



4. The cavities of the chest (thorax) and abdomen are 

 separated from one another by a muscular partition, which is 

 called the midriff or diaphragm, and is the chief agent in res- 

 piration. 



