THE MUTUAL RELATIONS. 313 



oxygen, the so-called " ozonide," which can be looked upon as a peroxide 

 of Isevulinaldehyde. 



O : C (CH 3 ) . CH 2 . CH 2 . CH : O 



O 



It decomposes into Isevulinaldehyde, Isevulic acid, and hydrogen peroxide, 

 on prolonged boiling with water. It is clear that, in the animal organism, 

 such an energetic action of oxygen can hardly take place on the unaltered 

 food materials, although it is possible that the cell might so change the 

 food as to make it more susceptible to attack. Such a conception corre- 

 sponds more nearly to the actual functions of the cells. It consumes the 

 substances which it needs from time to time; it must, therefore, have a 

 direct influence on these processes. Under normal circumstances, oxygen 

 is invariably present. The cell, i.e., its protoplasm, either activates the 

 oxygen, or else it seizes the separate food materials as it needs them, and 

 offers them to the oxygen for oxidation. At any rate, the decomposi- 

 tions, which have been observed directly, e.g., that of the carbohydrates, 

 may be traced back directly to cell-activity. The cell is, therefore, able 

 in two ways to regulate exactly its requirements of energy: by the activa- 

 tion of the oxygen, or by the preliminary preparation of the food for 

 combustion. 



We have intentionally gone into this matter somewhat in detail, in order 

 to show that the combustion of the food materials in the tissues is not 

 necessarily such a simple process as we have heretofore assumed. For 

 these reasons it is, as yet, impossible to decide whether fats are normally 

 converted into carbohydrates. Such a conversion is hardly probable. It 

 is more natural to imagine a direct utilization of the energy in fats by the 

 animal cell, and especially by the muscle cells. 



Experiments have been made with animals in the attempt to settle the 

 question whether fats are converted into carbohydrates. Before discussing 

 these we must show in general what deductions can be safely drawn from 

 such investigations. 



Investigations on the formation of sugars from other compounds than 

 carbohydrates, and especially from fats, have, as a rule, been carried out 

 in one of two ways. We can make the animal chosen for experiment free 

 from glycogen, by fasting, hard work, or strychnine convulsions, and then 

 feed it the compound which is to be tested as a glycogen-former. By 

 subsequently determining the glycogen content of the whole animal, it is 

 possible to find out whether any glycogen has been formed. Only in 

 exceptional cases, however, have these experimental conditions been satis- 

 fied. Rarely was it satisfactorily proved that the animal was free from 

 glycogen at the real beginning of the experiment, and such a source of 

 error is serious on account of the small amount of glycogen which is found 



