28 PHYSIOLOGY 



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 

 environmental 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 prevent 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 metabolism 

 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 appli- 

 cation 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 disproportion 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. 

 In this case the work performed by the muscle was 48 x 3 '8 = 182*4 

 grm. mm., while the potential energy of the stimulus represented only 

 0'5 x 10*0 = 5*0 grm. mm. Thus the work performed by the muscle 

 was thirty-six times larger than the energy of the stimulus applied 

 to the nerve. 



In the case of unicellular organisms, definite classes of motor 

 reaction to stimulus have been described. The ordinary retraction of 

 a unicellular organism, such as the vorticella, in response to a touch 

 is called thigmotaxis. Certain cells are influenced by gravity, tending to 

 rise or fall in the surrounding medium according to the conditions 

 which favour their existence. A similar sensitiveness to gravity is 



