CLASS MAMMALIA 



409 



exceeds in its development that of any other vertebrate type. The 

 olfactory lobes are well-developed and each of the two optic lobes is 

 divided, making four altogether. 



Carotid arter)^ 

 \Juguloir ]/ein 



Aortic arch 



Pulmonary 

 artery and 

 vein 



Subclavian 

 artery and 

 vein 



Hepatic 



portal 



system 



Vena cayc^ 



Iliac 

 artery 

 and vein 



ivfesenteric 

 vessels to 

 intestine 



Fig. 279. — Diagram of mammalian circulation. Vessels carrying venous blood, black; 



arterial blood, white. 



The eye is without a pecten, which was present in birds, and the 

 nictitating membrane is reduced in size, varying in its degree of develop- 

 ment among the different groups. A spiral coiling of the cochlea and 

 the possession of a chain of three bones in the middle ear, together with 



