5 i2 THE POPULAR SCIENCE MONTHLY. 



lism of the protoplasm of the special secreting organs and of the 

 general tissues. 



From the common nutritive fluid, the blood, protoplasm is 

 formed in all the tissues of the body, and we must look upon the 

 characteristic elements and products of these tissues as the result 

 of its destructive metabolism. In each organ of the body the 

 protoplasm appears to have special endowments adapted to their 

 specific functions, but these diverse activities are correlated to serve 

 a common purpose in the life of the individual. The contraction 

 of muscles, the specific secretions of the glandular organs, includ- 

 ing the salivary glands, the liver, the pancreas, the mammary 

 glands, etc., and, in fact, the products of all the metabolic tissues, 

 as well as their characteristic structural elements, must be consid- 

 ered as the resulting products of the downward steps of the me- 

 tabolism of protoplasm. 



As in plants the food elements are built up into protoplasm 

 before they are converted into the proximate constituents of plant 

 tissues (proteids, fats, starch, etc.), so in animal nutrition it ap- 

 pears that the proteids, fats, and carbohydrates, together with 

 oxygen introduced by the lungs, which constitute their food, must 

 pass through the intermediate phases of blood and protoplasm be- 

 fore they appear as animal proteids and fats, or enter into the 

 composition of the different tissues of the animal body, so that a 

 genetic or specific relation of particular tissues to special food 

 constituents can not be traced. 



For example, the muscles, from their comparative bulk, con- 

 tain a large proportion of the nitrogen of the body, and they are 

 spoken of as nitrogenous tissues, but they are not formed directly 

 from the proteid or nitrogenous constituents of the food. Like 

 all other tissues, they have their origin in protoplasm that is built 

 up from the common nutritive fluid, the blood, which is elabo- 

 rated, as we have seen, from the disintegrated elements of fats and 

 carbohydrates as well as proteids. Moreover, nitrogen is no more 

 essential to the formation of muscle than carbon or oxygen, or 

 even water, which are more abundant constituents of all living 

 tissues. 



It must then be evident that we can not formulate the propor- 

 tions of the proximate principles of foods that will serve the best 

 purpose in animal nutrition. The extended and profound series 

 of changes that intervene between the food constituents on the 

 one hand and the resulting animal tissues on the other are too 

 complex to enable us to trace any direct chemical relations be- 

 tween the initial elements and their final products. Aside from 

 these physiological considerations there are insuperable obstacles 

 in the way of prescribing diets that are even approximately suit- 

 ed to the requirements of any particular individual, or group of 



