306 VERTEBRATE ANIMALS. 
MAMMALIA. 
CHAPTER XXXII. 
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 ts always provided with hairs, 
and the young are nourished for a longer or shorter time by 
means of a special fluid—the milk—secreted by special glands— 
the mammary glands. ‘These two peculiarities are of them- 
selves 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 
¢wo articulating surfaces or condyles, instead of one, as in the 
Reptiles and Birds. 
2. 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 quadrate 
bone. 
3. The heart consists—as in Birds—of four distinct chambers, 
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. 121, @) are, in the great majority of cases, in the 
form of circular discs, and they never contain any internal solid 
particle or nucleus. 
4. The cavities of the chest (thorax) and abdomen are sepa- 
rated from one another by a muscular partition, which is called 
the midriff or diaphragm, and is the chief agent in respiration. 
