THE METABOLISM OF THE BODY. 443 



The Effects of Salts, Water, etc., in the Diet— When we 

 come to inquire as to the part salts play when introduced into 

 the blood, we soon find that our knowledge is very limited. 



Sulphur, and especially phosphorus, seem to have some im- 

 portant use which quite eludes detection. It is important to 

 remember that certain salts are combined with proteids in the 

 body, possibly to a greater extent than we can learn from the 

 mere analysis of dead tissues. 



Pathological. — The withdrawal of any of the important salts 

 of the body soon leads to disease, clear evidence in itself of their 

 great importance. This is notably the case in scurvy, in which 

 disease the blood seems to be so disordered and the nutrition 

 of the vessel-walls so altered that the former (even some of the 

 blood-cells) passes through the latter. 



Water. — The use of water certainly has a great influence 

 over the metabolic processes of the body. The temporary ad- 

 dition or withdrawal of even a few ounces of water from the_ 

 regular supply of a dog in the course of a feeding experiment 

 greatly modfles the results obtained for the time. It is well 

 known that increase of water in the diet leads to a correspond- 

 ing increase in the amount of urea excreted. It is likely that 

 even yet we fail to appreciate fully the great part which water 

 plays in the animal economy. 



THE ENERGY OF THE ANIMAI. BODY. 



As already explained, we distinguish between potential or 

 latent and actual energy. All the energy of the body is "to be 

 traced to the influence of the tissues upon the food. Energy 

 may be estimated as mechanical work or as heat, and the one 

 may be converted into the other. All the processes of the 

 organism involve chemical changes, and a large proportion of 

 these are of the nature of oxidations; so that speaking broadly, 

 the oxidations of the animal body are the sources of its energy, 

 and in estimating the quantity of energy, either as heat or work, 

 that a given food-stuff will produce, one must consider whether 

 the oxidative processes are complete or partial ; thus, in the case 

 of proteid food, if we suppose that the urea excreted represents 

 the form in which the oxidative processes end or are arrested, 

 we must, in estimating the actual energy of the proteid, sub- 

 tract the amount of energy that would be produced were the 

 urea itself completely oxidized (burned.) 



