CIRCULATION OF THE NUTRIENTS 109 



nutrients are absorbed by the blood vessels in the walls of 

 the alimentary canal. Since, however, many tissues of the 

 body are at a considerable distance from the organs of di- 

 gestion, it is evident that some means must be provided for 

 suppljring each cell with the nutrients it needs. This is 

 effected by the circulation of the blood. By the term cir- 

 culation of the blood is meant the ceaseless movement of the blood 

 through a system of tubes called blood vessels. 



154. Organs of circulation. — As is also true in the fish 

 and other vertebrates, the forbe that drives the blood around 

 through the body is largely furnished by the contraction 

 of the muscular walls of the heart. Any blood vessel that 

 carries blood away from the heart is called an artery.^ The 

 veins are the blood vessels that bring the blood back to the 

 heart. Connecting the arteries and the veins in every part 

 of the body are countless microscopic blood vessels called 

 capillaries (Latin, capillu^ = hair, so called from their mi- 

 nute size). We shall now consider in more detail the struc- 

 ture and action of each of these circulatory organs. 



III. The Heakt 



155. Position, size, shape. — The heart (Fig. 2) is a 

 conical or pear-shaped organ about the size- of the fist. It 

 lies behind the breastbone near the middle of the chest 

 cavity, with its pointed end or apex extending toward the 

 left side between the fifth and sixth ribs. Since the beat of 

 the heart is felt most plainly near the apex, it is commonly 

 but wrongly believed that the heart lies on the left side of 

 the body. Let one imagine the front wall of the chest 



' From Greek, aer = air + terein = to hold — a name which was 

 given by the early anatomists to these tubes, because they were 

 found empty after death, and were therefore supposed to carry air. 



