CH. LII.] VARIETIES OF CUTANEOUS SENSATIONS 727 



corpuscles per square centimetre; in other parts of the palm and 

 sole the number varies from 2 to 8. End-bulbs are believed to 

 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 like 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. 



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 

 physiological zero is altered : they persist until a new zero is formed, i.e. , until adapta- 

 tion is complete. So, too, heavy weights feel unduly heavy after light weights, and 

 vice versa. When eyeglasses or false teeth are first worn, their contact is well-nigh 

 unbearable ; yet later, through adaptation, the discomfort becomes negligeable. 



It is very difficult to draw any hard and fast line between the cutaneous sensa- 

 tions hitherto described, and those which are grouped under the name "common 

 sensibility." Sensations which are difficult to describe but which are perfectly 

 familiar, such as those accompanying tickling, shivering, shuddering, and the like, 



psychologists agree to place it under a " special 

 heading. 



The term "common sensation " is most frequently employed in reference to sensa- 

 tions from the interior of the body, and in this connection Head's work on the 

 relation of internal to cutaneous pain must be mentioned. To him we owe the 

 knowledge of the spatial relationship of the associated sensations, and this is of a 

 segmental nature. Each viscus stands in relation with a definite patch of skin ; 

 that is to say, the afferent fibres from the skin and from the viscera belong to 

 corresponding spinal nerve-roots. Localisation of painful or uncomfortable feelings 

 arising from disorders of internal organs is always very difficult ; hence the associated 

 skin pains play an important part in ascertaining the position of internal maladies. 



Drugs. Cocaine applied locally depresses all forms of cutaneous sensibility, 

 but especially the true tactile sense ; carbolic acid acts similarly but less strongly. 

 Chloroform produces a temporary burning sensation, and then blunts sensibility 

 especially to temperature changes. Menthol produces a feeling of local cold because 

 it first causes hyperaesthesia of the end organs for cold ; this is followed by a depres- 

 sion of the same end organs. 



