THE STRUCTURAL BASIS OF THE BODY 27 



draw any hard-and-fast distinction between these two sets of processes. 

 Assimilation requires the ingestion of food into the organism, and in 

 the second place its digestion, i.e. its solution in the juices of the cells. 

 These two processes are succeeded, through stages which we cannot 

 trace, by an actual growth in the living material. In naked cells 

 ingestion may occur either at any part of the surface, as in the amosba, 

 or at a specialised portion, so-called ' mouth,' as in many of the 

 infusoria. Digestion is apparently effected in most cases by the produc- 

 tion and secretion around the ingested food particle of solutions con- 

 taining ferments, i.e. agents which have the power of hydrolysing the 

 different food-stuffs and rendering them soluble. 



In the vast majority of living organisms the energy for their 

 activities is derived from the oxidation, ultimately of the food-stuffs, 

 but immediately of molecules attached to the living protoplasm. A 

 necessary condition, therefore, for the life of these cells is the presence 

 of oxygen in the surrounding medium, from which it is taken up in 

 the molecular form. We may therefore speak of an assimilation of 

 oxygen ; but it is still a matter of dispute whether the oxygen is built 

 up as such in the living molecule (so-called intramolecular oxygen) 

 to be utilised for the formation of carbon dioxide when a discharge 

 of energy is necessary, or whether it is only taken in at the moment 

 when the combustion of the carbon and hydrogen constituents of the 

 food or protoplasm is necessary for the supply of energy. However 

 this may be, products are formed as a result of this oxidation which 

 are of no further value to the cell and are therefore excreted, i.e. 

 turned out of the cell. The chief of these are the products of oxidation 

 of carbon and hydrogen, namely, carbon dioxide and water. There 

 are also many substances resulting from the oxidation of the nitro- 

 genous portions of the protoplasm, which have to be excreted in the 

 solid or dissolved form. 



Although the assimilation of oxygen is so general a quality of living proto- 

 plasm, the presence of this gas, at any rate in the free form, does not seem to 

 be necessary for all kinds of life. Thus a number of the bacteria are known 

 which are anaerobic, i.e. exist only in the absence of oxygen. Examples of 

 such are b. tetanus, and the bacillus of malignant oedema. In order to cultivate 

 them it is necessary to displace all the air in the cultivating vessels by means 

 of a current of hydrogen. It has been supposed that the ultimate source of 

 the energy of these organisms is also derived from a process of oxidation, and 

 that they differ from other organisms in being able to utilise for this purpose 

 oxygen which is built up into the structure of their food substances. It is 

 possible, however, that these organisms derive the energy for the building 

 up of their protoplasm, for their movements, &c., not from a process of oxidation 

 at all, but from processes of disintegration of the substances which they utilise 

 as food. It is by such means that in all probability the intestinal worms, fairly 

 highly organised animals, are able to exist in the intestine in a medium con- 

 taining no oxygen, but rich in carbon dioxide. Here they are plentifully supplied 

 with food-stuffs and can afford to adopt a wasteful method of nutrition, in which 



