294 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



water in the tissues and in the liquids of the body is naturally absolutely 

 essential to the normal play of metabolism ; and conditions, such as muscular 

 exercise, which increase the water-loss bring about also an increased water- 

 consumption, the regulation being effected through the nervous mechanism 

 which mediates the sensation of thirst. The water taken into the body does 

 not, however, serve directly as a source of energy, since it is finally eliminated 

 in the form in which it is taken in ; it serves only to replace water lost from 

 the tissues and liquids of the body, and it furnishes also the menstruum for the 

 varied chemical reactions which take place. Continued deprivation of water 

 leads to intolerable thirst, the cause of which is usually referred to the altered 

 composition of the tissues generally, including the peripheral nervous system. 

 Inorganic Salts. The essential value of the inorganic salts to the proper 

 nutrition of the body does not commonly force itself upon our attention, since, 

 as a rule, we get our proper supply unconsciously with our food, without the 

 necessity of making a deliberate selection. NaCl (common table-salt) forms 

 an exception, however, to this rule. Speaking generally, inorganic salts do 

 not serve as a source of energy to the body. Most of the salts found in 

 the urine and other excreta are eliminated in the same form in which they 

 were received into the body. Some of them, however, notably the phosphates 

 and the sulphates, are formed in the course of the metabolism of the tissues, 

 and without doubt reactions of various kinds occur affecting the composition 

 of many of the salts for example, the decomposition of the chlorides to form 

 the HC1 of gastric juice. But these reactions do not materially influence the 

 supply of energy in the body : the value of the salts lies in the general fact 

 that they are necessary to the maintenance of the normal physical and chem- 

 ical properties of the tissues and the body-fluids. Experimental investigation * 

 has shown in a surprising way how immediately important the salts are in this 

 respect. Forster fed dogs and pigeons on a diet in which the saline constit- 

 uents had been much reduced, although not completely removed. The animals 

 were given proteids, fats, and carbohydrates, but they soon passed into a 

 moribund condition. It seemed, in fact, that the animals died more quickly 

 on a diet poor in salts than if they had been entirely deprived of food. 

 Similar experiments were made by Lunin upon mice, with corresponding 

 results. He showed, moreover, that while mice live very well upon cow's milk 

 alone, yet if given a diet almost free from inorganic salts, consisting of the 

 casein and fats of milk plus cane-sugar, they soon died. Moreover, if all the 

 inorganic salts of milk were added to this diet in .the exact proportion in which 

 they exist in the ash of milk, the mixture still failed to support life. It would 

 seem from this result that the inorganic salts capnot fulfil completely their 

 proper functions in the body unless they exist in some special combination 

 with the organic constituents of the food. In this connection it is well to bear 

 in mind that proteids as they occur in nature seem always to be combined 

 with inorganic salts, and the properties of proteids, as we know them, are 

 undoubtedly dependent in part upon the presence of this inorganic constituent. 

 1 Bunge : Physiological and Pathological Chemistry, translated by Wooldridge, 1890. 



