THE CIRCULATION OF THE BLOOD 509 



countless number of tubes of capillary size, the 

 walls of which are so thin that substances in 

 solution pass through them with great ease. 

 These capillaries are the ultimate branches of 

 a single tube, and, after fulfilling their function, 

 the capillaries unite into a single tube again. A 

 closed system is thus formed. This system is 

 filled with a modified lymph called the blood, 

 which is kept in constant circulation. Thus the 

 lymph in the intervascular spaces is in intimate 

 contact with a continually changing liquid. 

 Further provision for rapid exchange is found 

 in the circulation of the lymph itself. The 

 spaces between the cells are drained by channels 

 which gradually become definite tubes, the lym- 

 phatics, and these finally join to form two ducts 

 which empty into the blood vessels. 



The unbranched portion of the vascular tube 

 is dilated into a cavity with thickened muscular 

 walls termed the ventricle of the heart. The 

 ventricle contracts rhythmically. Each contrac- 

 tion raises the pressure in the ventricle until it is 

 higher than the pressure in the remaining blood > 

 vessels. The blood in the ventricle is thereby \ 

 forced into the blood vessels against the resist- 

 ance of friction. The high pressure in the ven- 

 tricle during contraction is transmitted into the 

 blood vessels and through them. At each cross- 



