CONTROL OF THE CIRCULATION. 77 



Muscle Fibers in the Walls of the Lymph Tubes. - 



There are plain muscle fibers in the walls of the lymph 

 ducts. 



Lymphatic Glands. In its course the lymph passes 

 through many kernel-like masses, the lymphatic glands. 

 Lymph contains corpuscles which are considered identical 

 with the colorless blood corpuscles. It is tho'ught that 

 these corpuscles are formed in the lymphatic glands. 



The Flow of Lymph. The flow of lymph is partly 

 due to the blood pressure in the capillaries ; this pressure 

 is caused by the heart. (In the frog there are two small 

 hearts, not, however, near the blood-pumping heart, 

 and these pump the lymph along.) In our bodies the flow 

 of lymph is largely aided by any pressure that may be 

 brought to bear on the lymph veins ; for, on account of 

 the valves, as in the blood veins, any pressure must push 

 the liquid toward the heart. Thus the action of the mus- 

 cles in the limbs, in the chest, in the abdomen, in the 

 movements of breathing, and in the bending of the body, 

 etc., all help in this flow, which is always, probably, very 

 much slower than that in the blood veins. 



Relations of Blood Flow and Lymph Flow. It will 

 now be seen that while the blood leaves the left ventricle 

 by one tube, the aorta, it returns to the right auricle, not 

 merely by the two caval veins, but that a part of the blood 

 (i.e. of the liquid part of it) does not return by blood veins, 

 but having left the blood system proper through the thin 

 walls of the capillaries, it is brought back to the heart by 

 the lymph veins, which, however, join the blood veins just 

 before they empty into the heart. There is, in other 

 words, only one set of distributing tubes, but there are two 

 sets of collecting or returning tubes. 



