204 



ELEMENTARY BIOLOGY 



learned (see p. 148), excreted from the 

 lungs ; small amounts of urea, and pos- 

 sibly other organic wastes, also leave 

 the body through these organs. Some 

 water, salt, and urea, with traces of other 

 organic wastes, leave the body by way 

 of the sweat glands, and some reach the 

 intestines and are then thrown out. But 

 most of the waste is eliminated by way 

 of the kidneys. 



The kidneys, of which there are two, 

 may be considered as glands whose function 

 is the separation from the blood of certain 

 specific substances (as urea, salt, etc.) and 

 water (Fig. 78). 



237. The sweat glands. The sweat is 



excreted by special glands which consist 



of delicate twisted tubules surrounded by 



a network of capillaries (Fig. 'j'j). Waste 



material, with water and salt, is con 



stantly passing from the capillaries into 



the tubules, and through these out upon 



the surface of the skin. Ordinarily the 



water part of the perspiration evaporates 



as fast as it comes out of the glands, 



leaving a solid deposit of the wastes. 



When perspiration is more rapid, we can 



see the drops of sweat on the skin. When this dries, the solid 



deposit is left on the outside of the skin, instead of in the 



mouths of the tubules. 



Fig. 77. A sweat gland 



The sweat gland consists of 

 a fine tubule opening to the 

 surface of the skin at one 

 end and coiled up in a knot 

 at the other. The coiled 

 portion is surrounded by 

 blood vessels (capillaries) 

 from which water, urea, and 

 salts are withdrawn into the 

 gland tube 



