252 THE BODY AT WORK 



effective at the moment of their occurrence. When they 

 generate a succession of responses, it is because they continue 

 to produce changes in the protoplasm. Their continued 

 action does not, under ordinary circumstances, prolong the 

 response. 



Response to stimulation travels as an impulse through 

 protoplasm. An impulse is commonly likened to a wave, but 

 enough has been said already to prove that the simile is mis- 

 leading. It is not of the same nature as the wave which a 

 stone starts on the surface of a pond, a pulsation of sound 

 through air or water, an undulation of light or heat in the 

 aether. These various kinds of waves are waves of displacement, 

 a swing first to one side and then to the other. An impulse 

 traverses protoplasm, whether it be the apparently diffuse 

 protoplasm of a leucocyte or the severely oriented protoplasm 

 of a nerve or muscle, as a change which may be described as 

 chemical, with reservations as to the meaning allowed to this 

 term. We may without impropriety represent the fall (dis- 

 sociation) and subsequent rise (association) graphically as a 

 wave ; but even then it is but a half -wave, and inverted. It 

 is a very different thing to the onward progression of an 

 accession of force, with which it is not infrequently confused. 



All protoplasm is not equally susceptible of stimulation. 

 Probably it is safer to put this in a different form. Protoplasm 

 is not everywhere equally exposed to stimulation, nor is it when 

 especially exposed to stimulation in one way equally accessible to 

 all other effective forces. A sense-organ is a collection of cells in 

 which protoplasm is so disposed as to be susceptible to a certain 

 kind of stimulus. It is a " receptor " for a particular force. 

 At the same time it is essential to its efficiency that it should 

 be insusceptible to other forces. The protoplasm in certain of 

 the sense-organs of the skin dissociates when compressed, in 

 others when warmed. The cells of these receptors have a 

 certain structure which exposes their protoplasm in such a 

 manner that it cannot escape dissociation when, in the one 

 case, the cells are squeezed, or when, in the other case, they are 

 heated. The ear contains sensory cells so constructed that the 

 protoplasm which they contain dissociates when affected by 

 pulsations of sound. In the receptors of the tongue and 

 the nose protoplasm is exposed to the influence of chemical 



