MAMMALIA. 309 



MAMMALIA. 



CHAPTEE XXX. 



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 hy means of a special fluid — 

 the milk — secreted hy 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 tlie spinal column by means of ttoo 

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

 and Bii'ds. 



2. The lower jaw consists of two halves, each comjjosed of a 

 single piece, and united in front. The lower jaw, also, its always 

 jointed directly to the skull, and there is no quadrate bone. 



3. The heart consists — as in Birds — of four distinct chambers, 

 two aui'icles and two ventricles. The right and left sides of the 

 heart ai-e 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. 163, a), 

 are, in the great majority of cases, in the form of circular discs, and 

 they never contain aiiy internal 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 mid- 

 riff or diaphragm, and is the chief agent in respiration. 



.5. The respiratoiy organs are in the f<u-m of two lungs, placed in 

 the chest, and never communicating with air-receptacles situated in 

 different parts of the body. In no case and at no period of life are 

 gills or branehiffi present. 

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