385 



CLASS IV. 



MAMMA LI A. 



WE have now reached the class which ranks as the highest of 

 the animal kingdom; and to which man himself belongs. 

 Here only do we find organs especially adapted for supplying 

 to the young, during the prolonged period of helpless infancy, 

 that fluid nutriment, to which we give the name of milk. 

 This organization is so characteristic, that from the Latin 

 word mammce, signifying paps or teats, is derived the term 

 mammalia, the scientific appellation hy which the class is 

 distinguished. Every animal that suckles its young may, 

 from that circumstance, be referred to the present class. 



Circulation. The blood is warm, and the heart, as in birds, 

 consists of four compartments. The general arrangement of 

 the arteries through which the aerated blood in man is pro- 

 pelled, is shown in the annexed figure (286) which may be 

 compared with Fig. 241, exhibiting the arterial system in the 

 preceding class. 



" Neither the circulation nor the respiration are quite so 

 active, nor is the animal heat quite so great as in the class of 

 birds."* 



Eespiration. All the mammalia breathe by lungs. These 

 are not attached to the ribs as in birds, but are suspended in 

 a cavity at the upper portion of the trunk (thorax}. They 

 are divided into a multitude of minute cells into which air is 

 conveyed by the branches of the windpipe. In the annexed 



* Owen. 



