MAMMALIA. 



U.J 



The cavity of the aljdomen in Mammals is always sepai'ated 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 li.earl 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 aui'icle 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 lilood, which 

 has become impure by passing- 

 through the tissues, is returned by 

 the gi-eat veins to the rigid auricle, 

 from which it jmsses into the right 

 ventricle. From here it is dri\en 

 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 

 heai't by a series of vessels called 

 the pulmonary veins, and is poured 

 into the left auricle, from which it 

 jiasses into the left ventricle. From 

 the left ventricle it is propelled to 

 all parts of the body by a great 

 systemic vessel, wdiich is called the 

 aorta. 



The lumjf of Maran)als are two in number, and differ from those 

 of Birds in being freely suspended in memljranous liags. They are 

 spongy and cellular throughout, and they never comnnmicate by 

 apertures on their surface with air-sacs i>laced in ditierent parts of 

 the body. 



The nervoui t^ijiUm of Mammals is chiefiy rcnuirkable for the 



iy. '225. — Diagram of the cireiilatinn 

 ill a Mammal. (The cavities contain- 

 ing' venou.s lil'ioil are marked black, 

 those containini,' arterial blood are 

 left white.) ft Right anricle ; v 

 Right ventricle ; p Pnlmonai'y artery 

 carrying venous blood to the lungs ; 

 jtv Pulmonary veins carrying arterial 

 blood from the lungs ; a' Left auricle ; 

 v' Left ventricle ; h Aorta carrying 

 arterial bhiod to the body; c Vena 

 ca\'a, carrying venous blood to the 

 heart. 



