THE METABOLISM OP THE BODY. 435 



own knowledge, assume it gratuitously, in the absence of the 

 clearest' proofs, especially when our failures on this supposition 

 are so numerous. 



We may say, then, that fat is not merely selected from the 

 blood, but formed in the animal tissues ; that fat formation 



Fig. 326 —Microscopic appearances of— I, milk; II, cream; III, butter; IV, colostrum 

 of mare; V, colostrum of cow (after Thanhoffer). 



may take place when the food consists largely of carbohydrates, 

 when it is chiefly proteid, or when proteid and fatty. In other 

 words, fat results frona the metabolism of certain cells, which 

 is facilitated by the consumption of carbohydrate and fatty 

 food, but is possible when the food is chiefly nitrogenous. We 

 must, however, recognize diflferences both of the species and the 

 individual in this respect, as to the extent to which one kind of 

 food or the other most favors fat formation (excretion). The 

 use of the adipose tissue as a packing to prevent undue escape 

 of heat is evident ; but more important purposes are probably 

 served, as will appear from later considerations. 



Pathological. — Excessive fat formation, leading to the ham- 

 pering of respiration, the action of the muscles, and, to a certain 

 extent, many other functions of the body, does not arise in man 



