CHAPTER LXVII 

 THE RECEPTOR MECHANISMS CUTANEOUS SENSATIONS 



THE Receptors arc classified as Extero-ceptive : Non-Distance : 

 Touch, heat, cold, pain, taste; and Distance: Smell, sight, hearing. 

 Proprio-ceptive : labryinthine (semicircular canals), and kinaesthetic 

 (muscles, tendons, and joints). Entero-ceptive : those of common 

 sensibility (thirst, hunger, and pain). 



In regard to the extero-ceptive mechanism, certain general formulae 

 or laws have been formulated to describe the relations between ex- 

 ternal stimuli and conscious reactions. The most important of these 

 is known as the "law of specific sense energy," or " Miiller's law." 

 It states that (1) different stimuli acting upon the same sense mechan- 

 ism produce the same kind of sensation; (2) the same stimulus acting 

 upon different sense mechanisms calls forth different sensations. For 

 example, stimulation of the optic nerve, whether by the normal means 

 of stimulation (the so-called " adequate " stimulus), the vibrations 

 of the ether, or by abnormal mechanical means, such as a blow on 

 the eye (a so-called "inadequate" stimulus), evokes the sensation 

 of light. 



Another important law, known as Weber's law, states that " the 

 just noticeable increase of a stimulus bears a constant ratio to the 

 original stimulus," or " two stimuli, in order to be discriminated, 

 must be in a constant ratio, the latter being independent of the abso- 

 lute magnitudes of the stimuli." The actual value of this ratio, 

 although constant for any one sense mechanism, varies, from organ 

 to organ. For example, if one candle added to ten just perceptibly 

 increased the illumination, ten candles would have to be added to 

 a hundred to do so, and one hundred to a thousand. The validity 

 of this law has been hotly contested, since the experimental methods 

 employed to establish it have yielded very inconstant results. Un- 

 doubtedly, some relation of the kind enunciated by Weber's law does 

 exist, since all judgments involve comparison. 



Touch, Heat, Cold, Pain. With the exception of taste, the non- 

 distance receptor mechanisms mainly come under the class of 

 " cutaneous sensations," and are located in the skin. 



The Structure of the Receptors of Cutaneous Sensation. There is 

 a dearth of knowledge as to the structure of the receptor mechanism 

 concerned in the sensations of touch, heat, cold, and pain. In the 

 skin various nerve-endings have been described: 



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