Glass 6. Mammalia. 



489 



centrally^ elsewhere muscular : when the muscular portion contracts, 

 the diaphragm is flattened so as to enlarge the thorax, the very elastic 

 lungs are expanded, and consequently air rushes in : when the 

 diaphragm is relaxed, it is arched up into the thorax, the lungs are 

 compressed, and the air is partially expelled. The diaphragm is thus 

 an apparatus for effecting respiration. The muscles which 

 move the ribs are also of importance in this connection, since, when 

 the lower ends of the ribs are moved forwards, the thoracic cavity is 

 widened. 



In many Mammals ■wMcli liibernate (Bats, Hedgehogs, Mai-mots, Hamsters, 

 Myoxus), and in some otters (Mole, Shrew-mouse, Mouse, Rabbit), there is a 

 so-called hibernating gland, a lobed mass, reaching from the thorax to 

 the neck, over a portion of the back ; it is largest in autumn, and decreases in 

 size during winter. The organ consists of connective tissue with numerous large 

 cells containing oil-globules, and is to be regarded, like other fatty masses, 

 solely as a reserve of nourishment. 



Vascular System. As in Crocodiles and Birds, both atrium 

 and ventricle are completely divided, the left auricle receives 

 blood from the lungs, the right 

 from the rest of the body. The 

 pulmonary arteries (the 

 arterial arches of the fourth pair), 

 arise by a common stem from the 

 right ventricle. The arterial arches 

 of the first and second pairs 

 arise together by a common trunk, 

 the innominate artery, from 

 the left ventricle, and thus carry 

 simply arterial blood ; the right 

 arterial arch of the second pair 

 only gives off the artery for the 

 right limb ; the aorta (in contrast 

 to that of Birds) is formed by the 

 left arch of the same pair ; the 

 first pair of arterial arches forms 

 the carotids as usual. Thus in 

 Mammalia we have, as in Aves, a 

 complete separation of arterial and 

 venous blood. A conus arteriosus 

 is absent. 



ao 



In the new-bom Mammal there is 

 a large opening (forainen ovale) in the 

 septum between the two auricles, which 

 is, however, soon closed. In the same 

 way there is an open duct (the ductus 



Botalli) between the second left arterial arch and the pulmonai-y artery 

 (the f oui'th ai-terial arch) ; after bu-th it degenerates to a solid cord. 



398. Heart and arterial arches 

 of Mammalia, diagrammatic. a right, 

 a' left auricle, ao aorta, s and s' snb- 

 clayian arteries, v right, v' left ventricles, 

 1, 1', 2, 2', 4, 4' arterial arches. — Orig. 



