METABOLISM, NUTRITION, AND DIET. 447 



property of protoplasm to separate from the blood the materials which 

 it may require to produce secretions, in the case of the protoplasm of 

 secreting glands, or to enable it to evolve heat and energy as in the 

 case of the protoplasm of muscle. The properties of the protoplasm are 

 very possibly differently developed in each case, and the decomposition 

 products, too, may be different in quality or quantity. Proteid materials 

 appear to be specially needed, as is shown by the invariable presence of 

 urea in the urine even during starvation ; and as in the latter case there 

 has been no food from which these materials could have been derived, 

 the urea is considered to be derived from the disintegration of the nitro- 

 genous tissues themselves. Which, if not all, of the three varieties of 

 proteid of the blood, viz., serum-albumin, serum-globulin, and fibrino- 

 gen, is necessary for muscular metabolism is not certainly known, 

 opinion appears to incline toward the first as the most important. The 

 removal of all fat from the body in a starvation period, as the first appar- 

 ent change, would lead to the supposition that fat is also a specially 

 necessary pabulum for the production of protoplasmic energy; and the 

 fact that, as mentioned above, with a diet of lean meat an enormous 

 amount appears to be required, suggests that in that case protoplasm 

 obtains the fat it needs from the proteid food, which process must be 

 evidently a source of much waste of nitrogen. The fat which is 

 deposited in the tissues has for its origin, as we have before remarked, 

 in great part carbohydrate food, and is looked upon as a store of carbo- 

 naceous material; it has been suggested that as it leaves the tissue to be 

 used up, it is reconverted into a carbohydrate, viz., dextrose. Salts 

 appear to be absolutely essential for protoplasmic life. The idea that 

 proteid food has two destinations in the economy, viz., to form organ or 

 tissue proteid which builds up organs and tissues, and circulating pro- 

 teid, from which the organs and tissues derive the materials of their 

 secretions or for producing their energy, is a convenient one, but cannot 

 be said to rest upon any very certain facts. Except in the possible case 

 of the appearance of leucin and tyrosin in pancreatic digestion, already 

 fully discussed, it must not be looked upon as more than a convenient 

 hypothesis. 



One question which has been little considered by physiologists, is 

 what relationship, if any, there is between each tissue and the waste pro- 

 ducts of other tissues, or perhaps it should be said, the products of the 

 metabolism of other tissues. It is not known whether, as the result of 

 ;he katabolism of one tissue, products, proteid or otherwise, are not 

 taken up by the blood and carried to other tissues, supplying exactly 

 svhat is necessary for their complete anabolism ; whether, for example, 

 i proteid residue does not arise from the metabolism of muscle which 



