24 NUTRITIONAL PHYSIOLOGY 



who sees the various parts of a watch the wheels, springs, 

 jewels, etc. lying loose upon the table of the watch- 

 maker. He may gain a fair notion of the intricacy of the 

 watch, though he may be very far from knowing how the 

 parts were related in the time-piece. We do well to use 

 the plural number in speaking of the proteins, for all recent 

 work tends to emphasize the distinctive molecular pat- 

 tern which characterizes each form and makes it differ 

 definitely from every other. " There is one flesh of men 

 and another of beasts and another of fishes and another of 

 birds." This is excellent and altogether modern chemical 

 biology. The presence of the element nitrogen in the 

 proteins distinguishes them sharply from the other promi- 

 nent compounds in both the body and its daily income. 

 Nitrogen makes up about 16 per cent, of protein, and the 

 value of this figure in calculations will be apparent later. 

 It has been said that the proteins are second only to water 

 in their abundance in the body. This is not true of the 

 diet unless we have to do with a carnivorous animal. 

 Among the herbivora, and almost always among men, the 

 second place jnjthe list of supplies is occupied by the 

 carbohydrates. 



Third in quantity among the constituents of the body 

 we find in most individuals the mineral compounds. 

 These would not make so large a proportion if it were 

 not for the_skeleton. _Bone is a tissue in which salts of 

 lime are abundantly present. But in all the other tissues 

 and in the fluids too we find a variety of salts, and it is 

 well established that their presence is not accidental, but 

 ' a matter of moment. Sodium, potassium, calcium, and 

 magnesium at least, perhaps other bases also, must be 

 kept in certain balanced relations if the life processes are 

 to go on. The acids represented are chiefly hydro- 

 chloric, phosphoric, and carbonic. Sodium chlorid, 

 the one salt which we take pains to add to our food, 

 is the one most abundant in blood and lymph. Potas- 

 sium rather than sodium compounds predominate in the 

 cells. 



