GENERAL PHENOMENA OF NUTRITION. 85 



stant formation of fresh tissue to take the place of that which 

 has been destroyed by use ; and in this essentially consists 

 the nutrition of an organism in its adult condition. Every 

 organism, then, is compelled to Be incessantly manufactur- 

 ing fresh matter fit to replace the losses caused by vital 

 action ; and the power by which this is effected is known by 

 the general name of assimilation. With the exception, 

 namely, of parasites living on the already elaborated juices 

 of their hosts, no organism takes as food materials which 

 can be built up directly and without change into new tissue. 

 On the contrary, the materials taken as food have to 

 undergo certain changes before they can be employed in 

 repairing loss they have to be " assimilated " or made like 

 to the tissue which they are to replace. 



The power of assimilation is one of the most remarkable 

 of the properties of living matter; and it is one which 

 resides, not in the organism as a whole, but in each in- 

 dividual portion and every separate tissue of the organism. 

 However simple may be the being with which we have to 

 deal, and even if there be no special alimentary apparatus, 

 the general result of the digestive process is the production 

 of a common nutritive fluid, which contains certain organic 

 compounds manufactured out of the food during the process 

 of digestion. In the case of the higher animals, this com- 

 mon nutrient fluid is called the blood, and there is usually a 

 special organ or " heart," by which it is propelled through 

 special tubes, or " blood-vessels," to every organ and every 

 tissue in the body. Many of the lower animals are destitute 

 of any such special apparatus ; but in all cases the nutrient 

 fluid which is the result of the digestive process, and which 

 corresponds with the blood of the higher forms, is distributed 

 to all parts of the organism. The blood, however, or the 

 nutrient fluid which takes its place, is simply a solution of 

 certain organic compounds, and the assimilation of these 

 compounds is effected in the tissues themselves, the part 

 played by the blood being the merely passive one of serving 



