PART VI 

 THE SENSE-ORGANS 



SECTION XX 

 SPECIAL SOMATIC AND VISCERAL RECEPTORS 



CHAPTER LIX 

 CLASSIFICATION OF THE SENSE-ORGANS 



The DijBferent Manifestations of Energy. — Until now we have re- 

 garded the central nervous system as a mechanism controlling our 

 various motor actions in a way to conform with the conditions existing 

 in our environment. As such it occupies a position intermediate 

 between the different afferent and efferent paths, fulfilling the function 

 of a machine which converts or synthetizes the incoming impulses into 

 motor responses. We have previously seen that the efferent side of 

 this circuit is not greatly diversified, because all the effectors in our 

 body are built up either of muscle tissue or glandular tissue. On the 

 afferent side, on the other hand, we find a decided multiplicity in 

 structure to correspond with the diverse character of the influences 

 to which we may be subjected. 



Our body is surrounded by a medium which is teeming with mani- 

 festations of energy in its different forms. Against these the body 

 is partially protected by a relatively impervious capsular investment, 

 the skin. Here and there, however, this investment is beset with 

 orifices for the admission of those stimuli which are essential to our 

 life. These perforations really correspond to the windows of a house 

 which enable us to come into functional relationship with the outside 

 world. This is also true of the lower animals and even of those 

 forms which are not in possession of a nervous system. In the latter, 

 the different manifestations of energy are brought to bear directly 

 upon their hving substance, but not in an indiscriminate manner, be- 

 cause, with the exception of a few, these organisms are all at least 

 partly protected against excessive stimuli by some sort of an enveloping 

 membrane or calcareous shell. Furthermore, all these organisms pos- 

 sess a much more limited range of excitation than the higher forms, so 

 that they are open to only a few types of stimuli, principally to those 

 of mechanical and thermal origin. 



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