524 MANUAL OF ZOOLOGY SECT. 



communication. Each half consists of an auricle and a 

 ventricle, opening into one another by a wide opening, 

 guarded by a valve composed of three membranous cusps 

 on the right side, two on the left. The right ventricle gives 

 off the pulmonary artery ; the left gives off the single aortic 

 arch, which passes over to the left side, turning round the 

 left bronchus in order to run backwards as the dorsal aorta. 

 The blood is warm. The red blood-corpuscles are non- 

 nucleated and usually circular. 



The two cerebral hemispheres, in all but the monotremes 

 and marsupials, are connected together by a band of trans- 

 verse fibres, the corpus callosum, not represented in the 

 lower vertebrates. The dorsal part of the mid-brain is 

 divided into four optic lobes, the corpora quadrigemina. 

 On the ventral side of the hind-brain is a transverse band of 

 fibres, the pons Varolii, by which the lateral portions of the 

 cerebellum are connected together. 



The ureters, except in the Prototheria, open into the 

 bladder. 



Mammals are all, with the exception of the monotremes, 

 viviparous. The fcetus is nourished before birth from the 

 blood-system of the parent through a special development 

 of the fcetal membranes and the lining membrane of the 

 uterus, termed the placenta. After birth the young mam- 

 mal is nourished for a longer or shorter time by the milk or 

 secretion of the mammary glands of the parent. 



The class Mammalia is divisible into two main divisions 

 or sub-classes, the Prototheria and the Theria. 



The Prototheria are mammals in which the mammary 

 glands are devoid of teats ; the oviducts are distinct through- 

 out, and there is a cloaca into which the ureters and the 

 urinary bladder open separately. In the centra of the 

 vertebrae the epiphyses are absent or more imperfectly de- 



