24 CLASS I. MAMMALIA. 



than in those which are carnivorous, as the changes to be 

 produced in the nature of the aliment are greater. 



The chyle, being mixed with the blood, becomes a part of 

 it, although it is not known where or how its change from the 

 white to the red color is effected. It is then circulated 

 throughout the body, by the heart, the arteries, and the veins. 

 The heart is a hollow muscular organ, the main-spring of the 

 circulation ; the arteries are long cylindrical canals or pipes, 

 carrying the blood from the heart to the different parts of the 

 body ; the veins are vessels of a similar form and structure, 

 bringing the blood back to the heart, after it has gone the 

 round of circulation. 



The heart -in rnan is a double organ ; that is, it consists of 

 two complete and distinct organs, united together into one 

 mass, but performing their functions without interference or 

 connection. These two parts are called the right and left 

 sides of the heart ; and each has two distinct cavities, called 

 auricles and ventricles. The right side of the heart receives 

 the blood from the body at large, and sends it to the lungs ; 

 the left receives it from the lungs, and sends it to the body. 

 The heart is of a conical shape, is situated in the thorax, just 

 within the sternum, a little inclining to the left side. It is, 

 however, placed with the apex, or point of the cone, extend- 

 ing downwards and to the left, so that it touches the ribs at 

 the spot where the beating is felt, and hence has usually been 

 supposed to lie entirely on the left side. The main body of 

 the heart is composed of the two ventricles, which are strong 

 muscular cavities, the left far more so than the right ; the 

 auricles are situated around the base of the organ, seeming 

 rather to be loose appendages than constituent parts of it. 



We shall begin with the course of the blood at the point 

 where it receives its new supply from the chyle. The sub- 

 clavian vein, after uniting with the vein from the other arm, 

 and the veins coming down from the head and neck, conveys 

 its blood immediately to the right auricle, where it meets 

 with that brought from the lower parts of the body. The 

 two trunks, which bring the venous blood in this way to the 

 heart, are called the descending and ascending venae cavae. 

 They pour their blood into the right auricle, which contracts 

 and expels it, through an opening for that purpose, into the 

 right ventricle. This opening is guarded by valves, which 

 prevent the flowing back of the blood, by completely closing 

 the passage. When the ventricle has become distended, 

 it contracts in its turn, and the blood, being prevented by the 







