CHAPTER LXXXVII 



THE PROPERTIES OF EACH PART OP THE REFLEX ARC 



Having briefly traced the physiological development of the nervous sys- 

 tem, we are prepared to consider in greater detail the peculiar function 

 of each of the parts which enter into the formation of the reflex arc. 



THE RECEPTOR 



With the advance in animal organization is associated the development 

 of the ability to appreciate and discriminate between external phe- 

 nomena, special organs called receptors being evolved to receive the 

 stimuli which these occasion. Those receptors which are distributed 

 over the skin of the animal are called external or cxteroceptors, and are 

 especially adapted to react to such stimuli as temperature, pressure, 

 and pain, but at the fore end of the animal certain receptors become more 

 highly specialized so as to react to stimuli coming from a distance — 

 that is, to stimuli that are not produced by contact of external objects 

 with the surface of the animal. These specialized receptors — sometimes 

 called proficient — include the eye, the ear, and the olfactory epithelium. 

 Receptors are also provided in the interior of the organism for the pur- 

 pose of receiving stimuli dependent upon the activities of the organism 

 itself. They may be called internal receptors, and we may further dis- 

 tinguish two groups of them — namely, those which come from the sur- 

 faces of the mucous membranes and those which come from the sub- 

 stance of the various organs and tissues themselves, as, for example, 

 from the substance of muscle or tendon. 



A receptor may be denned in a general way as a mechanism in which 

 some particular kind of stimulus produces changes that result in the 

 excitation of the nerve fiber with which the receptor is connected, al- 

 though the stimulus in itself is incapable of exciting the nerve fiber. In 

 other words, as Sherrington puts it, the receptor has the threshold of 

 its excitability raised to every kind of stimulus save one, toward which 

 it is lowered. A nerve fiber, for instance, responds to every kind of 

 stimulus approximately equally; a receptor will also respond to these 

 same stimuli, but with great inequality, since each receptor is specialized 

 to react to one kind of stimulus and to others only when these are very 

 strong. 



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