CHAPTEK in. 



THE USE AND NATURE OF SENSE ORGANS. 



Heat and light are physical influences to which even 

 the lowest units of living matter respond, whether their 

 mode of life and nutrition be most akin to that of Plants 

 or to that of Animals. These influences act upon such 

 organisms, either by stimulating, retarding, or otherwise 

 modifying the chemical changes occurring in their interior, 

 and upon the existence of which their Life depends. 



Where the vital processes of the organism are stimulated 

 by these physical agencies, their incidence may, in many 

 instances, become the cause of so-called * spontaneous * 

 movements. And some sort of foundation exists for this 

 popular mode of expression. A movement which follows 

 immediately upon some localized external stimulus is 

 not said to be * spontaneous : ' the term is generally 

 applied where the cause of the movement is not distinctly 

 recognizable. In some of these cases — as when we have 

 to do with the influence of a diffused physical agent such 

 as heat — an undetected or unconsidered external cause 

 really exists, which, by stimulating the vital processes, 

 gives rise to movements seemingly spontaneous. Whilst in 

 other cases, movements apparently spontaneous are to 

 be referred to internal states or changes, that is to impres- 

 sions emanating from some of the internal organs which, 

 after passing through one or more ganglia, are trans- 



