MAMMALIA. 



285 



right ventricle. From here it is driven through a great vessel, 

 called the pulmonary artery, to the lungs, where it is submitted 

 to the action of the air, and becomes arterial blood. It is then 

 returned to the heart by a series of vessels, called the pulmo- 

 nary veins, and is poured into the left auricle, from which it 

 passes into the left ventricle. From 

 the left ventricle it is propelled to 

 all parts of the body by a great 

 systemic vessel, which is called the 

 aorta (Fig. 141). 



The lungs of Mammals are two 

 in number, and differ from those of 

 Birds in being freely suspended in 

 membranous bags. They are spongy 

 and cellular throughout, and they 

 never communicate by apertures on 

 their surface with air-sacs placed in 

 different parts of the body. 



The nervous system of Mammals 

 is chiefly remarkable for the great 

 proportionate development of the 

 brain, as compared with the spinal 

 cord. 



In the higher Mammals, also, 

 the two halves (hemispheres) of the 

 brain proper (cerebrum) are con- FIG. 141. Diagram of the circulation 

 nected together by a great band or 



bridge of nervOUS tiSSUe, COnstltut- 



ing what is known as the corpus 

 callosum. This structure is not a 

 conspicuous feature in the two low- 

 est orders of the Mammalia. The 

 senses, as a rule, attain great per- 

 fection in the Mammals; and the 

 only sense which can ever be said to be entirely wanting is 

 that of sight. Eyes, however, are always present, though 

 they may be rudimentary ; and in those Mammals which are 

 said to be " blind," it is not generally that the eyes are want- 

 ing, but that the skin passes unbrokenly over the eyeball, or 

 the optic nerve is degenerated. Even in these cases, however, 

 it is not impossible that there maybe some perception of light 

 through the skin. An external ear for collecting sounds is 

 usually present; but it is wanting in the Whales and Dol- 

 phins, and in some of the Seals. 



black, those containing arterial 

 blood are left white.) a Eight 

 auricle; v Eight ventricle ; p Pul- 

 monary artery carrying venous 



blood to the lungs ; pv Pulmonary 

 veins carrying arterial blood from 

 the lungs ; a f Left auricle ; t/ Left 



ventricle ; & Aorta carrying arte- 

 rial blood to the body ; c Vena cava 

 carrying venous blood to the heart 



