208 BIOLOGY 



While passing through these capillaries, the food materials, 

 absorbed by the blood frtrni the alimentary canal, and the gas 

 absorbed from the lungs, pass from the blood into the tissues 

 where they are needed. In this way the food and oxygen are 

 supplied to the active tissues of the body. At the same time 

 waste products, which have been produced in the active tissues, 

 are returned to the blood, so that the blood, after passing through 

 the capillaries, goes back to the heart as impure blood. After 

 reaching the heart the impure blood goes to the lungs, where 

 part of its impurities are passed off into the air. 



Lymph System. A part of the circulatory system is called the 

 lymph system. As the blood is flowing in the capillaries some 

 of the liquid plasma soaks through the walls of the capillaries 

 out into the tissues. When it reaches the tissues it is no longer 

 called blood but lymph, and is a colorless clear liquid which 

 bathes every living cell. This lymph contains, dissolved in it, 

 the nutriment absorbed from the intestine; and, since it now 

 actually bathes the living cells, these can take from it directly 

 the nourishment they need for their activities. Into this lymph 

 the living cells also excrete all the waste products that have 

 resulted from their life processes, the lymph receiving all the 

 wastes of the body. The gases, which comprise part of this 

 waste, pass at once into the blood by diffusion; but the other 

 materials remain dissolved in the lymph and finally reach the 

 blood by the following course: The lymph gradually collects 

 in tiny spaces, lacunae, scattered over the body, and from these 

 flows into little vessels connecting with each other, called lymph 

 vessels. These small vessels unite together to form larger ones 

 and the larger vessels finally empty into the veins. The vessels 

 around the front end of the body converge to two minute sacs 

 lying deeply imbedded near the third vertebra; and the vessels 

 in the hind part of the body converge into similar sacs situated 

 over the hips, near the lower end of the urostyle. These four 

 sacs have muscular walls and pulsate, and are called lymph 

 hearts. When they beat they force the lymph into the veins 



