LYMPH DUCTS. 99 



In dissecting the frog, the looseness of the skin is very 

 noticeable. The large spaces under the skin are lymph 

 spaces. Sometimes considerable lymph is found here, so that 

 in holding up a frog the sagging of the skin from the weight 

 of the lymph may be easily seen. 



There are valves where these lymph ducts empty into the 

 veins, which prevent any reflow of liquid into the ducts, but 

 allow the lymph to pass freely into the veins. There are 

 plain muscle fibers in the walls of the lymph ducts. 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 at all, 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. (See Fig. 37.) 



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 cor- 

 puscles. It is thought that these corpuscles may be formed 

 in the lymphatic glands. 



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 muscles 

 in the limbs, and in the chest and abdomen, the movements of 



