FUNCTIONS OF QtJADRUPEDS. 13 



generally four, five, six, or seven spleens, and many pan- 

 creatic glands. 



The Organs of Respiration and Circulation. 



:'!'' 



Iii the bodies of all mammiferous animals there are three 

 principal cavities ; the head, the thorax or chest, and the 

 abdomen. Of these it is the chest which contains the organs 

 necessary to respiration and circulation. This cavity is 

 separated from the abdomen by a strong mernbranaceous par- 

 tition, furnished with muscular fibres, called the diaphragm. 

 It contains the heart, the lungs, *the trachea, or windpipe, 

 and the oesophagus, or gullet. 



The heart is situated in front of the chest, betwixt the 

 two lobes of the lungs. It is composed of muscular fibres, 

 and is the engine by which the blood is driven through the 

 arteries for the support and nourishment of the body. Its 

 basis, from which the great vessels arise, is generally covered 

 Avith fat; and it has two hollow and fleshy appendages, 

 called auricles. It contains two cavities, divided from each 

 other by a fleshy septum. One of these is denominated the 

 right, and the other the left ventricle ; but in the human 

 subject, where the position of the heart is different from that 

 of quadrupeds, the ventricles might be more appropriately 

 denominated anterior and posterior. From the contraction 

 of the left ventricle the blood is forced into the arteries. 

 These carry it to the remotest parts of the system ; and it is 

 conveyed back to the heart by another set of vessels, deno- 

 minated veins, through their common trunk, the vena cava, 

 which terminates in the right auricle. This last commu- 

 nicates with the right ventricle by an opening, where are 

 valves disposed in such a manner as to permit the blood to 

 enter, but not to return. At the bottom of the aorta, or 

 trunk of the arteries, there are three valves, which open only 



upwards 



