774 CUTANEOUS SENSATIONS [CH. LIII. 



be the organs for cold; they are most numerous in the conjunctiva 

 and glans penis, where " cold spots " are almost exclusively present. 

 The end-organs in "heat spots" have not been identified with 

 certainty, but they are probably larger organs, and placed more 

 deeply in the skin. 



We have spoken of the pressure sense as the true tactile sense ; 

 but Meissner pointed out many years ago that the hand immersed in 

 a fluid such as mercury at body-temperature, does not feel the contact 

 of the fluid, although the fluid pressure may be far above the limen ; 

 it is, however, equal in all directions ; it is therefore clear that the 

 adequate stimulus for touch organs consists in a deformation of the 

 skin surface. 



As compared with the sensation obtained from pain spots, touch 

 is quicker both in development and subsidence. Thus vibrations of 

 strings are recognisable as such by the finger, even at a frequency 

 of 1500 vibrations per second. A revolving wheel with toothed edge 

 does not give a sensation of smoothness till the teeth meet the skin 

 at the rate of from 480 to 640 per second. 



Head, in his recent study of nerve-regeneration, cut one of the 

 nerves in his own arm, and, in conjunction with Eivers, noted 

 accurately the date and other particulars of return of function. The 

 first sensations return about the eightieth day after the operation ; 

 they are termed by him protopathic. Protopathic sensibility depends 

 on definite specific end-organs distributed over the skin as sensory 

 " spots," viz., heat, cold, and pain spots. When this sensibility is 

 alone present, the spaces between these spots are insensitive to 

 cutaneous stimuli ; the heat spots only react to temperatures above 

 37 C., the cold only to temperature below 26 C. ; the sensation 

 radiates widely, and is often wrongly localised. The tactile sensa- 

 tions of the skin, the intermediate temperature sensations, the power 

 to localise them accurately, the sensibility of the spaces between the 

 spots, and a more refined sensibility to pain, return much later, 

 and this epicritic sensibility was not perfect until many months 

 after the regeneration started. As previously stated (p. 706), it 

 is not known whether protopathic and epicritic impulses are sub- 

 served by the same or by different nerve-fibres. Quite apart from 

 these two forms of cutaneous sensation is the deep sensibility of 

 subjacent structures, and the fibres subserving this run mainly with 

 the motor nerves ; this form of sensation is not destroyed by division 

 of all the nerves to the skin (see also p. 705). 



Adaptation plays a part as important in cutaneous as in other sensations. The 

 same room feels warm to a man who enters it from the street, and cold to another who 

 has been in a conservatory. Hering calls the point of adaptation to temperature ' ' the 

 physiological zero." Thus the temperature of the mouth and the lips may actually 

 differ by several degrees, yet neither of them will feel hot or cold because each is at 

 the physiological zero temperature. Sensations of warmth or cold arise when the 



