288 



Elementary Biology. 



FIG. 152. CAPILLARY NETWORK 

 UNITING AN ARTEK>' AND A VEIN 

 IN THE WEB OF THE FROG'S FOOT. 

 (Quain.) 



epithelial cells only. Every artery subdivides into capillaries ; 

 every vein is formed by the reunion of capillaries ; the 

 current of the circulation is from the artery to the vein 

 through the capillary network so formed. Capillaries thus 

 form the ultimate subdivisions of the circulatory system, and 



as such bring oxygen and 

 nutritive fluid into close rela- 

 tion with the tissues, and at 

 the same time extract the 

 waste products produced in 

 the tissues as a consequence 

 of metabolic changes taking 

 place there. 



Having now briefly noticed 

 the general character of the 

 constituents of the vascular 

 system it will be necessary to 

 sketch the course of the chief 

 blood-streams through the 

 body. A little consideration 

 will show us that there must be three important currents, 

 viz. (i) a blood-stream from the alimentary canal which 

 passes through the liver to the heart, carrying the products 

 of assimilation into the general circulation ; (2) a blood- 

 stream from the heart to and from the various organs of the 

 body ; and (3) a blood-stream to and from certain purifi- 

 catory organs the lung and the kidneys. 



The first current is conveyed by a large vein, the portal 

 vein, which on the one hand has its terminal capillaries 

 diffused in the walls of the intestine, and on the other 

 breaks up into a similar capillary network in the substance 

 of the liver, in which organ the constituents of bile are ab- 

 stracted from the blood, while at the same time glycogen is 

 removed and stored in the liver-cells. This blood-stream 

 is known as the portal circulation. 



The second or systemic circulation is more elaborate. 



The arrows indicate the direction of 

 the blood-flow. 



