92 THE CELL 



inanimate objects. It must never be forgotten that there is no 

 substance in inanimate nature which remotely approaches the 

 living cell for complexity of structure, and that hence the reactions 

 of such a substance are of necessity correspondingly complex in 

 character. 



The field of the phenomena of irritability is exceedingly wide, 

 since it embraces all the correlations which take place between the 

 organism and the outer world. The stimuli which act upon us 

 from without are innumerable. For the sake of clearness, we will 

 consider them under five heads : (1) thermal stimuli, (2) light 

 stimuli, (3) electrical stimuli, (4) mechanical stimuli, (5) the 

 almost infinite variety of chemical stimuli. 



The manner in which an organism responds to one of these 

 stimuli is called its reaction. This may vary very considerably 

 with different individuals even when they are exposed to the same 

 stimulus. It depends entirely upon the structure of the organism, 

 or upon its finer properties, although these may not be perceptible 

 to us. Different organisms, to use a simile of Sachs (IV. 32a), may 

 in this respect be compared with variously constructed machines, 

 which, when set in motion by the same external force, heat, pro- 

 duce different useful effects according to their internal structures. 

 Similarly, the same stimulus may produce quite different effects in 

 different organisms, according to their specific structure. 



We shall see later on that many protoplasmic bodies are to 

 a certain extent attracted, whilst others are repelled, by light ; 

 a similar difference will be seen when the action of chemical 

 reagents, etc., on protoplasm is studied. The terms positive and 

 negative heliotropism, positive and negative chemotropism, galvanotro- 

 pism, and geotropism are used to describe these varying effects. 



Another phenomenon, in some respects the exact opposite of the 

 ones described above, must also be explained by the varying 

 specific structure of the stimulated substance ; the term specific 

 energy has been used to describe this phenomenon. Whilst, as 

 described above, we see that protoplasmic bodies, differing in 

 structure, react in various ways to the same stimulus, we find, on 

 the other hand, that similar effects are produced upon the same 

 protoplasmic body by very different stimuli, such as light, 

 electricity, or mechanical injury. 



A muscle cell responds to all kinds of stimuli by contracting, a 

 gland cell by secreting ; an optic nerve can only experience the 

 sensation of light, whether stimulated by light waves, electricity, 



