486 APPLIED BIOLOGY 



in which blood goes back to the heart. Lymph, then, os- 

 moses from blood-capillaries into lymph spaces, then flows 

 into lymph-capillaries, and through larger vessels back into 

 the blood. The arrangement of blood and lymph- vessels 

 in any organ might be compared with the irrigation systems 

 for watering agricultural land. The blood-capillaries corre- 

 spond to the ditches from which water seeps out into the 

 spaces between particles of soil. These spaces in the soil 

 correspond to lymph-spaces, while the lymph-capillaries and 

 larger tubes correspond to the drain tiles which carry excess 

 water away from irrigated soil. In short, the lymph-spaces 

 in any organ constitute a sort of combined irrigation and 

 drainage system for cells which are not directly reached by 

 the main canal system of the blood-capillaries. 



Lymph contains white corpuscles, which are able to squeeze 

 through the walls of blood-capillaries into lymph-spaces. 

 They are also formed in great numbers in the lymphatic 

 glands through which lymph flows on its way back to the blood 

 in the large veins. In fact, all the white corpuscles in blood 

 are lymph-cells washed into the blood from the lymphatic 

 glands. The spleen and the tonsils are examples of large 

 lymphatic glands, and there are hundreds of small ones in 

 various parts of the body. 



CIRCULATION OF THE BLOOD 



409. Need of Movement of Blood. In order that blood 

 may serve its purpose as a distributor of food and oxygen to 

 the cells and in removing excretions from the cells to the 

 excretory organs, it is necessary that it should be kept con- 

 stantly in motion. This is accomplished by the circulation 

 of the blood from the heart through the arteries and capil- 

 laries into the veins, which conduct it back to the heart. 

 That the blood thus goes around in a circuit was unknown 

 until 1621, when Dr. Harvey, of England, proved that in 



