VERTEBRATA 



177 



CLASS VI. Mammalia 



Mammals are distinguished from all other vertebrates 

 by any one of the following characters : they suckle 

 their young; the thorax and ab- 

 domen are separated by a perfect 

 diaphragm ; the red corpuscles 

 of the blood have no nucleus, 

 and are therefore double concave 

 (Fig. 259), and either a part or 

 the whole of the body is hairy at 

 some time in the life of the ani- 

 mal (Fig. 291, 30 1 ). 44 



They are all warm-blooded ver- 

 tebrates, breathing only by lungs, 

 which are suspended freely in 

 the thoracic cavity ; the heart is 

 four-chambered, and the circula- 

 tion is double, as in birds (Fig. 

 273); the aorta is single, and 

 bends over the left bronchial 

 tube ; the large veins are fur- 

 nished with valves ; the red cor- 

 puscles differ from those of all 

 other vertebrates in having no 

 nucleus and in being circular (ex- 

 cept in the camel) ; the entrance 

 to the windpipe is always guarded 

 by an epiglottis ; the cerebrum is 

 more highly developed than in 

 any other class, containing a 

 greater amount of gray matter 

 and (in the higher orders) more convolutions ; the cere- 

 bellum has lateral lobes, a mammalian peculiarity, and 

 there is a corpus callosum and a pons varolii (Fig. 335, 

 DODGE'S GEN. ZOOL. 12 



FIG. 164. Longitudinal section 

 of human body (theoretical) : a, 

 cerebro-spinal nervous system; 

 b, cavity of nose; c, cavity of 

 mouth; d, alimentary canal; 

 e, chain of sympathetic ganglia; 

 /, heart; g, diaphragm. 



