MAMMALIA. 



315 



The cavity of the abdomen in Mammals is always separated from 

 that of the thorax by a complete muscular partition — the diaphragm. 

 The abdomen contains the greater part of the alimentary canal, the 

 liver, pancreas, kidneys, and other organs. The thorax contains 

 chiefly the heart and lungs. The heart is contained in a membranous 

 sac — the pericardium — and consists of two auricles and two ven- 

 tricles. The heart consists functionally of two sides, each having 

 an auiicle and a ventricle, which 

 communicate with one another by 

 apertures, so guarded by valves 

 that the blood can pass from the 

 auricle into the ventricle, but not, 

 under ordinary circumstances, from 

 the ventricle to the auricle. There 

 is in the adult no direct communi- 

 cation between the two sides of the 

 heart. The course of the circula- 

 tion is indicated in the subjoined 

 diagram (fig. 225), and is shortly as 

 follows ; The venous blood, which 

 has become impure by passing 

 through the tissues, is returned by 

 the great veins to the right auricle, 

 from which it passes into the 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 pulmonaiy 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. 



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 



Fig. 225. — Diagram of the circulation 

 in a Mammal. (The cavities contain- 

 ing venous blood are marked black, 

 those containing arterial blood are 

 left white.) a Right auricle ; v 

 Right ventricle ; j) Pulmonary artery 

 carrying venous blood to the lungs ; 

 jiu Pulmonary veins carrying arterial 

 blood from the lungs ; a' Left auricle ; 

 «' Left ventricle ; 6 Aorta carrying 

 arterial blood to the body ; c Vena 

 cava, carrying venous blood to the 

 heart. 



