ELEMENTARY VITAL PHENOMENA 161 



cells. Under certain circumstances, however, the cells also retain 

 a small part of the proteid dissolved in the blood, either employing 

 it for the increase of its living substance, as in growth, or storing 

 it up in the protoplasm, as in fattening, in the form of reserve, 

 i.e., passive, proteid which is not ordinarily consumed in meta- 

 bolism. Under certain conditions, as in fasting or during the 

 development of eggs, such passive, indifferent, reserve-proteid 

 can again be drawn into the metabolism. The vitellin in egg-cells 

 is such a substance. 



Regarding the fate of the ingested fats and carbohydrates as 

 few details are known as regarding the finer transformations of 

 proteids. The fat which is taken as such into the cells frequently 

 remains for a long time as reserve-material. Likewise the fat that 

 is split up into glycerine and fatty acids and resorbed can be 

 changed back into neutral fat in the cell ; this is proved by the 

 striking experiments of J. Munk ('84), who, by feeding fat-free 

 soaps or free fatty acids, was able to cause a storing-up of 

 tissue-fat in dogs that had fasted and become extremely lean. In 

 a similar manner the grape-sugar that is split off from the carbo- 

 hydrates can be transformed synthetically into glycogen in the 

 tissue-cells, especially in the cells of the liver and the muscles, 

 and can be stored up as such. Regarding the further fate of this 

 stored fat and glycogen, however, it is known only that they can 

 be consumed during fasting and during excessive muscle-work, 

 that, therefore, they represent a reserve-material which acts in 

 cases of need as " compensation-food " in Pfltiger's sense. 



b. Dissimilation 



Our knowledge of the processes of dissimilation of living sub- 

 stances is much more meagre than that of assimilation. We 

 really know only that living substance is continually undergoing 

 decomposition, for this is apparent from the output of decompo- 

 sition-products. But as to the path from the complex proteid 

 compounds to the end-products, as to the special chemical trans- 

 formations that take place, our knowledge is very incomplete, 

 since as yet the composition of proteids is known very slightly. 



But one fact at least is certain, namely, that the most of all those 

 substances that result from the decomposition of the proteid mole- 

 cule are not groups of atoms that were preformed as such in the 

 molecule and are now simply split off, but they are derived from 

 certain cleavage-products by successive syntheses ; this takes place 

 either at the moment of decomposition by rearrangement of the 

 atoms in the proteid molecule itself, as in the case of carbonic acid, 

 or later outside of the proteid molecule by combination with other 

 cleavage-products and a simultaneous rearrangement of atoms, as is 

 the case, e.g., in the formation of uric acid. Thus far it is not known 



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