CHAP. Il] THE MAMMALIA 23 



X. The lungs are freely suspended within the cavity of the 

 thorax. 



XI. The heart is completely divided into two halves — a right 

 and a left — between which there is no aperture of communication. 

 Each half consists of an auricle and a ventricle, opening into one 

 another by a wide aperture, 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 : it 

 therefore represents the left aortic arch of Reptiles. 



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

 nucleated and usually circular. 



XIII. The two cerebral hemispheres, in all but the Proto- 

 theria and Metatheria, 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 marked by four 

 eminences— the corpora quadrigemina. On the ventral side of 

 the hind brain is a transverse band of fibres — the pons varolii — 

 by which the lateral parts of the cerebellum are connected 

 together. 



XIV. The ureters (except in the Prototheria) open into the 

 bladder. Mammals are all, with the exception of the Monotremes, 

 viviparous. 



XV. The foetus (except in the Prototheria) is nourished before 

 birth from the blood-system of the parent through a special 

 development of the foetal membranes and the lining membrane of 

 the uterus, termed the placenta. After birth the young mammal 

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

 of the mammary glands of the parent. 



Such are the characteristics common to all mammals. The 

 animals presenting these characters have been classified as 

 follows. 



