256 ANIMAL BIOLOGY 



not live nearly so long if he is deprived of water. Men 

 under the hot and dry climate of the desert crave a large 

 amount of water and soon succumb if they cannot obtain 

 it. 



In both our food and drink we consume small amounts 

 of various kinds of salts. Comon table salt or sodium 

 chloride, NaCl, is one of the most common of these, 

 and while it occurs in small quantities in animal foods and 

 often in drinking water, it is usually added as seasoning 

 to much of the food we eat. Carbonate and sulphate of 

 lime are common in drinking water, the so-called hard 

 water containing an unusually large amount of one or 

 both of these salts. These, with phosphate of lime, are 

 used in the formation of bone, as well as in supplying 

 calcium to other tissues of the body. Salts of potassium 

 and magnesium are also important, and iron is required 

 for the formation of the red coloring matter of the blood. 

 While salts are needed only in small quantities, they are 

 absolutely essential for the maintenance of life; the pres- 

 ence of several different kinds of salts is of even greater 

 importance than the consumption of different classes of 

 foods. 



There are several substances which are consumed not so 

 much on account of their value as food, but because they 

 gratify the sense of taste. Spices add to the piquancy of 

 various dishes, but they have practically no value as food 

 and, although they may serve a good purpose by stimulating 

 the appetite, some of them may produce bad effects, espe- 

 cially when taken in large amounts. There are several 

 substances called stimulants which may or may not be of 

 value as food. A stimulant is a substance that increases 

 the metabolic activity of the organism. Frequently the 

 excitement produced by the stimulant is followed by a 

 period of depression in which the vital energy of the body 



