THE MUTUAL RELATIONS. 311 



The animal organism has only a certain amount of room in its tissues for 

 the carbohydrates. The quantity of glycogen which can be stored up in 

 the liver, muscles, and the other organs, is very limited. The animal 

 organism is often provided with larger amounts of carbohydrate than it 

 is able, at the moment, to utilize. It is here that the depots for fat storage 

 are utilized. Large amounts of carbohydrates after having been trans- 

 formed into fat can be held in reserve until needed. We have not the 

 slightest idea where, in the animal organism, or in what organ, this trans- 

 formation takes place. It is possible that the liver executes this com- 

 plicated process. 



Although it is generally admitted that the animal cell is able to convert 

 carbohydrates into fats, the reverse process is a much disputed problem. 

 We are accustomed to look upon most chemical reactions as reversible. 

 We also know that the animal cells are capable, directly or indirectly, of 

 performing characteristic processes. They build up and they tear down 

 material only to reconstruct it, and, finally, by its complete destruction, 

 they utilize the energy contained in the food. We have seen representa- 

 tives of all three classes of nutrient materials break down in the intestine, 

 and, again, we have traced their reconstruction into more complicated 

 compounds. The cells of the liver produce glycogen from grape-sugar, 

 permitting it to become glucose again as it is required. Another problem 

 is this: Is the animal organism, under normal circumstances, capable of 

 satisfying its carbohydrate requirements from fats. This would hardly 

 be the case under ordinary conditions, for, a priori, as we shall later on see 

 more in detail, there is no reason at hand why the fat should first go 

 over into carbohydrate, in order that the organism may utilize its energy 

 for specific purposes. On the other hand, the possibility of such a trans- 

 formation unquestionably exists. We can imagine that each cell can only 

 utilize certain compounds for specific functions. Thus, we may assume 

 that the muscle cells operate only by means of carbohydrates. The selec- 

 tive action of the ferments would support such an assumption. We know 

 that, in many cases, they are unable to act upon substances closely related 

 to compounds that they can decompose. Every ferment seems peculiarly 

 fitted to attack only certain definite compounds. Thus it would be. easy 

 to understand that the muscle cells, which are especially adapted to act 

 upon carbohydrates, can utilize only such potential energy as is presented 

 to them in -this form. On the other hand, we must remember that the 

 cleavage and combustion of the food materials are not the source of the 

 liberated energies, but only act as a loosening momentum, or as a shock. 

 The true cause is the chemical energy of the food substances ultimately 

 the sun's energy, transformed sunlight. We cannot understand, a priori, 

 why the energy liberated by the combustion of the fats could not be 

 just as well utilized by the muscle cells as that which arises from the 



