26 PHYSIOLOGY 



carbon dioxide and water. There are also many substances resulting from 

 the oxidation of the nitrogenous 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 protoplasm, 

 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 bacillus tetanus, and 

 the bacillus of malignant O3dema. In order to cultivate them it is necessary to dis- 

 place 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 sub- 

 stances. 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 containing 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 only a small fraction of the energy 

 is obtained which would be produced by a total oxidation of the food. 



B. The Phenomena of Dissimilation. The activities of a living cell 

 or organism can be regarded in every case as dependent originally on en- 

 vironmental change, and are adapted to this change, i.e. are of such a nature 

 that they tend to preserve the organism intact, to favour its growth, or pre- 

 vent its destruction. The property of reacting in such a manner to changes 

 in the environment is fundamental to all protoplasm and is spoken of as 

 excitability, and the change which will influence an organism and cause a 

 corresponding adaptive change in it is known as a stimulus. Stimuli may be 

 of various kinds. Thus mechanical, thermal, chemical, electrical changes, 

 light, and so on, may act as stimuli. The reactions which they evoke involve 

 in every case chemical changes in the protoplasm, i.e. changes in the metabol- 

 ism of the cell. Sometimes this change may be assimilatory in character, 

 leading to an increased growth of the protoplasm, or at any rate to a cessation 

 of dissimilation. In such a case the stimulus is spoken of as inhibitory, 

 because it diminishes or prevents the output of energy by the organism. 

 The frequent result of a stimulus is an increased output of energy, which may 

 appear in the form of movement, in the form of heat, or as chemical change. 



A common feature of all dissimilatory changes evoked by the application 

 of a stimulus is that the energy of the reaction is always many times greater 

 than the energy represented by the stimulus, the excess, of course, being 

 supplied at the expense of the potential energy of the food material which 

 has been stored up in or built up into the living protoplasm. This dispropor- 

 tion between stimulus and reaction can be well illustrated on an excitatory 

 tissue such as muscle. Thus in one experiment the gastrocnemius muscle of a 

 frog was loaded with a weight of 48 gms. The nerve running to the muscle 

 was placed on a hard surface and a weight of half a gramme was allowed to 

 fall upon it from a height of 10 mm. The muscle contracted in response to 

 this mechanical stimulus applied to the nerve and raised the weight 3-8 mm. 



