862 CENTRAL NERVOUS SYSTEM 



intermediate temperature the sensation of warmth will be felt in the 

 former finger and of cold in the latter. The sense organs thus become 

 adapted to the conditions to which they are exposed, and become ex- 

 cited again only by a change in these conditions. 



Adaptation is a quite general sensory phenomenon. It occurs prom- 

 inently in the retina, with the result that an eye which has become used 

 to the dark is dazzled momentarily by the ordinary daylight. Adapta- 

 tion is also a marked feature of the touch sense, as is the experience of 

 every one who has worn flannel underclothing or a plate of false teeth. 

 It serves to protect the organism from the fatiguing effects of over- 

 stimulation from conditions to which it is continually exposed. Sensory 

 adaptation must be borne in mind not only when examining the senses 

 of others, but in making judgments with our own sense organs. Thus if 

 the hands are cold, the skin of another may feel warm and feverish, even 

 though its temperature is not above normal. 



Pain is of particular importance to the physician, whose services are 

 judged by the community largely by his skill in suppressing this sen- 

 sation. Unlike the other receptors, the pain spots are not specialized for 

 the detection of any particular form of stimulus, but may become excited 

 by any condition which threatens to harm the tissues. Pain may be pro- 

 duced by excessive pressure, the caustic action of chemicals, excessive ex- 

 posure to light as in sunburn, temperatures above 45 C. and the effects 

 of extreme cold, as in frost bite. The threshold for these stimuli is high, 

 so that in the strengths at which they are ordinarily experienced the 

 sensation of pain does not arise. All stimuli of sufficient intensity to 

 threaten the welfare of the tissues and give rise to pain are called 

 noxious stimuli. It was thought at one time that pain resulted from 

 the overstimulation of any type of receptor. This conclusion was natural 

 when one considered the great variety of conditions which might give 

 rise to painful sensations. Undoubtedly the overstimulation of any 

 sense organ may produce sensations of unpleasant character, but that 

 specific sense organs for pain exist can no longer be doubted. This is 

 shown by the presence of pain spots in regions from which other sense 

 organs are absent, as the cornea, the absence of pain in regions in which 

 other sense organs occur, as the inside of the cheek opposite the sec- 

 ond molar, by the existence of special tracts in the central nervous sys- 

 tem for the conduction of the impulses which give rise to pain, and by 

 the fact that certain drugs such as cocaine may abolish the excitability 

 of the receptors for pain without disturbing the reception of other 

 sensations. 



Pure sensations, arising from the stimulation of a single type of re- 

 ceptor, rarely occur in life except under the artificial conditions which 

 we employ in the laboratory. The sensations which we experience are 

 the composite result of the simultaneous combination of a variety of 



