HOW THE ANIMAL LIVES 211 



the blood the secretion which that gland is ap- 

 pointed to furnish, and pour it out through the 

 gland ducts. The secretion from one gland is 

 nutritious, as in the case of milk ; that from 

 another is digestive, as in the secretion of the 

 stomach; and from a third it is waste matter, like 

 sweat. The selection from the nutritive liquid of 

 the blood is the work of the individual cells, and 

 is always the same for each kind of gland. 



364. The cells of some glands construct a new 

 substance, which is not secreted but poured back 

 into the blood. Thus the liver makes glycogen, 

 which passes into grape sugar, and serves for 

 the production of heat, muscular work and nu- 

 trition. 



365. Some cells on the walls of the intestines 

 absorb nutritive and other matters from the 

 liquid contents of the bowels and pass them on 

 into the circulating (blood and lymph) vessels. 



366. Besides these cells which become im- 

 prisoned in their particular tissues, and the work 

 of which is restricted to the conducting of the 

 growth or other functions of such tissues, there 

 is a large class which floats free in the liquids 

 of the body. The red and white blood glob- 

 ules and lymph cells are examples. These 

 globules or corpuscles circulate in all parts of 

 the body, thus suggesting the freedom of the 

 one -celled animal. But limitations have beeu 



