CHAP, iv.] FEELING AND TOUCH. 577 



spot being cold after the cooling agent has been removed, that is 

 at a time when the cooled spot is in reality being heated by the 

 surrounding warmer tissues. 



The change in temperature of the skin necessary to produce a 

 sensation must have a certain rapidity ; and the more gradual the 

 change the less intense the sensation. The repeated dipping of the 

 hand into hot water produces a greater sensation than when the 

 hand is allowed to remain all the time in the water, though in the 

 latter case the temperature of the skin is most affected. The 

 effects of contrast are also seen in these sensations as in those of 

 pressure. 



We can with some accuracy distinguish variations of tempera- 

 ture, especially those lying near the normal temperature of the 

 skin. These sensations, in fact, follow Weber's law, though 

 apparently sensations of slight cold are more vivid than those 

 of slight heat, the range of most accurate sensation seeming to lie 

 between 27 and 33. 



The regions of the skin most sensitive to variations in tempera- 

 ture are not identical with those most sensitive to variations 

 in pressure. Thus the cheeks, eyelids, temples and lips, are more 

 sensitive than the hands. The least sensitive parts are the legs, 

 and front and back of the trunk. 



The simplest view which can be taken with regard to the 

 distinction between pressure sensations and temperature sensations, 

 and which is suggested by the facts just mentioned, is to suppose 

 that two distinct kinds of terminal organs exist in the skin, one of 

 which is affected only by pressure, and the other only by variations 

 in temperature ; and that the two kinds of peripheral organs are 

 connected with different parts of the central sensory organs by 

 separate nerve-fibres. Certain pathological cases have been quoted 

 as shewing not only that this is the case, but that the two sets of 

 fibres pursue different courses in the spinal cord. Thus in certain 

 diseases or injuries to the brain or spinal cord, hypersesthesia 

 as regards temperature has been observed unaccompanied by an 

 augmentation of sensitiveness to pressure ; and conversely instances 

 have been seen where the patient could tell when he was touched, 

 but could not distinguish between hot and cold. On the other 

 hand there are facts which shew a close dependence between the 

 sensations of pressure and temperature. When each stimulus is 

 brought to bear on a very limited area, the two sensations are fre- 

 quently confounded, especially in those regions of the body where 

 sensations are not acute. So also a penny cooled down nearly 

 to zero and placed on the forehead will be judged by most 

 people to be as heavy or even heavier than two pennies of the 

 temperature of the forehead itself; and conversely a body warmer 

 than the skin will often appear heavier than a body of the same 

 weight but of the same temperature as the skin. Moreover cases 

 have been recorded where a hot body, such as a heated spoon, 



F. 37 



