CIRCULATORY SYSTEM 



87 



now known as blood. Furthermore the body wall is 

 rigidly held in position by a firm heavy shell or case, and 

 thus mere bodily movements are no longer sufficient to 

 shift the body fluid; hence a muscular pump, the heart, 

 is present to drive it along a fairly definite route. In 

 many of the larger species there are vessels leading from 

 the heart to various parts of the body, which are thus 

 more directly supplied with ma- 

 terials from the digestive tract and 

 respiratory organs, but sooner or 

 later the blood pours through the 

 open ends of these arteries into 

 the main body cavity and is thus 

 brought into direct contact with 

 the various tissue cells. 



Circulatory System of Higher 

 Animals. — In the highest group 

 of animals, the vertebrates or 

 those with backbones, the trans- 

 portation or circulatory system is 

 an extensive and complicated set 

 of vessels communicating with a 

 compact and powerful pump, the 

 heart (Fig. 16). The exact course 

 of the blood through these chan- 

 nels is not an important matter 

 in this connection, but it may be 

 said that the one, or two, great 

 vessels or arteries leaving the 

 heart branch repeatedly as they 

 make their way into the tissues and 

 finally communicate with the cap- 

 illaries. These are exceedingly 

 slender and delicately walled vessels, which, it is im- 

 portant to note, are so numerous and widely distributed 

 that they are in the vicinity of practically every cell of 

 the body. On the other hand, the capillaries unite with 



Fig, 16. — Diagram of 

 circulatory system of 

 higher animals. A artery, 

 H heart, L lung, V veins. 

 Lymphatics in stippled 

 outline. 



