CHAPTER XXVI. 



THE NERVOUS SYSTEM (Cont'd). 

 Reflex Action. 



The Nerve Structure Involved in the Reflexes of the Higher 

 Mammals. In general, as already mentioned, these include a 

 receptor, an afferent fiber, a nerve center, an efferent fiber and 

 an effector organ. 



THE RECEPTOR. The receptor exists as one of the sensory 

 nerve terminators situated in the skin (extero-ceptors) or in the 

 deep tissues, such as the joints, the muscles or the viscera 

 (proprio-ceptors). Many receptors are highly specialized so as 

 to respond only to one kind of stimulus, and- each special kind 

 of receptor is located where, it will be of most use. Thus, there 

 are special receptors for sensations of heat, others for cold, others 

 for touch, others for pain. The pain receptors are distributed 

 more or less uniformly over the body. They are present in the 

 deeper structures, such as the teeth, the joints and the serous 

 coverings of the viscera. Sometimes, as on the cornea and in the 

 pulp of the teeth, they are the only kind of receptor present. 

 The touch receptors are collected in small areas called "touch 

 spots," which are much more numerous on the tip of the tongue, 

 the lips, or the tips of the fingers than on the skin of the legs, 

 the arms or the back of the trunk. The frequency of touch spots 

 on the tip of the tongue makes a foreign body in the mouth ap- 

 pear to be larger than when we feel it with the fingers. The 

 touch spots on the finger tips may acquire great acuity of per- 

 ception by education, as in the case of a blind person, who has 

 to use his fingers for reading. The remarkable irregularity of 

 distribution of touch spots may be very beautifully shown by 

 finding out how far apart the points of a pair of calipers must 

 be from one another in order to be distinguished as separate. 

 This distance is not more than 3 mm. for the tips of the fingers, 



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