638 LECTURE XXVII. 



requires for the exercise of its functions. It retains its own cell-material. 

 The fact that now and then a few cells are broken down and renewed, 

 hardly affects the metabolism as a whole. 



This very simple representation of metabolism is, however, as we shall 

 find on closer examination, not entirely in accordance with certain facts. 

 As we have already repeatedly stated, it has become recognized during the 

 last few years that the digestion of the food is not for the sole purpose of 

 making it capable of absorption. Unquestionably, one of the principal 

 objects is to make the material which is obtained from different sources 

 conform to the material out of which the body is made up. The substances 

 contained in the food are, by means of the various ferments, not only made 

 capable of absorption, but of assimilation as well. Starch, which is the 

 glycogen of plants, is broken down into molecules of d-glucose, only to be 

 changed later into animal glycogen. We do not yet know how much of 

 the absorbed glucose is changed into the latter compound, nor whether 

 the glycogen throughout the entire animal kingdom is all of the same nature, 

 or whether perhaps there are not different kinds of glycogen corresponding 

 to the different species of animals. Here we cannot decide definitely 

 whether the preliminary preparation is all-important as regards assimilation, 

 or whether it merely serves the purpose of making the material capable of 

 absorption. The question is similar in the case of the fats. In this case 

 one might even get the impression that the nutriment is deposited in an 

 unchanged condition. The fat is split by the digestive ferments into its 

 constituents, fatty acids and glycerol, which, however, unite again in the 

 intestinal wall. It has been found possible to cause foreign fat to be 

 deposited in the body. We have already shown that the fat stores occupy 

 a peculiar position in animal economy. It is a question whether the animal 

 organism can also utilize substances foreign to it for physiological functions 

 in cell-metabolism. At the same time we may quite safely assume that 

 in the case of the fats, digestion only serves to prepare it for absorption. To 

 be sure, we must confess that the fat stores of different animals under 

 normal conditions are not homogeneous as regards their composition. 

 How far these differences in chemical composition depend upon differences 

 in the nature of the food, remains an open question. In general we may 

 indeed assume that the fat contained in the stores has a specific composi- 

 tion for every animal, in case the animal is not deprived of the power to 

 construct its own fat by decomposition and selection, on account of being 

 limited to fat of a definite kind. 



At all events, it is very remarkable how quickly and easily the animal 

 organism decomposes, by means of the ferments, the fats and complicated 

 carbohydrates into their simple components, only to rapidly reconstruct 

 them again, and eventually, at the time they are consumed, carry away 

 the resulting products equally rapidly. These very complicated processes 



