THE METABOLIC MACHINERY OF \NIMALS 277 



larger amounts. In the case of man the amount of water in tlie adult 

 body remains approximately the same under normal conditions, but 

 if decreased beyond a certain point intolerable thirst results. On the 

 other hand, if the amount of water is increased, the blood pressure is 

 raised in the renal capillaries and the excretion of urine is stimulated. 

 The consumption of a hberal supply of water is a characteristic 

 biological process as it favors the removal or dilution of waste and 

 poisonous materials from the body. 



Along with water, the presence of certain chemical elements such as 

 sodium, potassium, calcium, magnesium, iodine, iron, chlorine, phos- 

 phorus, sulphur, silicon, and fluorine is necessary to maintain the 

 various kinds of tissues. Much experimental work has been per- 

 formed upon various animals, indicating the importance of a proper 

 balance of these elements in the diet. The absolute withdrawal of 

 any of these may end in the death of the organism. 



Since these salts form, a part of all tissues and serve a variety of 

 functions it is impossible to mention all of them. The important 

 part w^hich calcium salts, for example, play in the formation of 

 bone is well realized. In this connection it has been said that 

 there is enough lime in a human body " to whitewash a small hen- 

 coop." 



Certain parts of foods rich in carbohydrates contain indigestible 

 material that serve as roughage and are useful in stimulating the 

 muscles of the large intestine. Bran, whole wheat, fresh vegetables, 

 and fruit provide some of the best sources of these materials. Other 

 examples may be found in the cellulose of plant cells which can be 

 used as food by only a few animals. 



Flavorings, stimulants, and condiments, such as pepper, mustard, 

 tea, coffee, and cocoa, are not true foods. However, they have a 

 real value in making food more appetizing. 



Vitamins 



It might seem that an organism could be kept alive, well, and 

 healthy upon a balanced diet of the necessary inorganic salts and 

 water, together with energy producers and tissue builders, such as 

 amino acids, carbohydrates, and proteins. Modern scientific work 

 has dispelled this illusion by a series of laboratory experiments and 

 by observations of experiments performed in nature. We now know 

 that regulating substances, called vitamins, are some of the most 

 essential ingredients of all foods. 



