194 BIOLOGY AND HUMAN LIFE 



in plants are various kinds of pigments, insoluble crystals, tan- 

 nins (commonly found in the bark of trees and in unripe fruit), 

 various acids, and alkaloids, gums, and resins. 



The one-celled animals excrete their wastes directly into the 

 surrounding water. In the higher animals, those that have 

 blood and lymph, the wastes are diffused from the living cells 

 into these conducting fluids and are then eliminated from the 

 body through special organs. 



161. The kidneys. In man and the other backboned animals 

 the kidneys are the special excretory organs. Water and car- 

 bon dioxid, as we have already learned (see page i6i), are ex- 

 creted from the lungs, as well as small quantities of urea and 

 possibly other organic substances. Some water, salts, and urea, 

 with traces of other organic w^astes, leave the body by way of 

 the sweat glands (Chap. XX) ; and a certain amount of waste 

 matter gets into the intestine directly through the lining cells, 

 in part by way of the white corpuscles (p. 179) and in part 

 through the secretions of the liver. From the intestine these 

 substances are removed, together with the refuse from the 

 food, in the feces. But most of the wastes given off by the body 

 cells are taken into the blood and filtered out by the kidneys. 



There are two kidneys, each about as long as the width of 

 your hand and bean-shaped. They are located in the back of 

 the abdominal cavity, a little lower than the stomach. The 

 structure of the kidney is that of a gland, a mass of tiny tubules, 

 branched and twisted, with a complex network of capillaries. 

 The waste substances diffuse through the walls of the capillaries 

 into the tubules, and the fluid (urine) is gathered by these 

 tubules into a funnel-shaped hollow (see d, Fig. 98). 



162. Hygiene of the kidneys. The kidneys work constantly, 

 and their continuous operation is essential to the health of the 

 body as a whole. The whole system would be quickly poisoned 

 if the wastes were allowed to remain in the blood or the cells. 

 Since the kidney removes wastes in solution, an abundance 

 of water is a necessary part of the daily income. We can do 

 little more toward maintaining the health of the kidneys than 



