30 PHYSIOLOGY CHAP. 



(d) Sensibility to heat is not uniform in the extremities ; 

 some distant parts are more sensitive than others more proximal. 



(e) The lateral surfaces of the extremities are less sensitive to 

 heat than the mesial sides. 



To these conclusions we may add that in those cutaneous 

 regions which are peculiarly adapted to tactile sensibility (as 

 the hand in general, the tips of the fingers in particular) the 

 thermal sensibility to both the cold and heat sense is less than in 

 other regions. 



Parts that are habitually covered are more sensitive to cold 

 than exposed parts. This is not due entirely to habit, but 

 principally to the fact that the covered parts contain a great 

 many cold spots : for the same reason the skin of the face, though 

 it is constantly exposed, is not less sensitive to cold than the 

 covered parts of the skin. 



The terminal apparatus of the thermal nerves has in common 

 with other nervous organs the property of being more strongly 

 excited in proportion as the stimulation is more rapid. As the 

 adequate stimulus consists in the addition or subtraction of heat 

 at the thermal points, it may be said that the excitation or 

 reaction of the latter is more intense in proportion as the increment 

 or decrement of heat occurs more rapidly. 



The strength of the sensation also depends partly upon the 

 extent of cutaneous surface excited. A thermal stimulus dis- 

 tributed over a large area of skin evokes a stronger sensation than 

 a stimulus of the same strength acting on a smaller area. This is 

 easily demonstrated by plunging one finger of one hand and the 

 whole of the other hand into water ; or by dipping one finger into 

 water at 40 C. and the other hand into water at 37 C. In both 

 experiments the sensation of warmth is less in the finger than in 

 the hand. Weber also noted that a stimulus which is purely 

 thermal when applied to a small surface may become painful if it 

 acts on a larger surface. One finger alone can be plunged into 

 water at a temperature at which the immersion of the whole limb 

 would be painful. 



The reaction time for sensations of cold and that for tactile 

 sensations are equally short ; on the other hand, the reaction time 

 for sensations of heat, as that for sensations of pain, is longer 

 (Tanzi). According to Kiesow and Ponzo, the reaction time for 

 heat is shortened if stimuli that penetrate the skin more readily 

 than those employed by Tanzi are adopted, and if the specific 

 points are excited directly. Nevertheless, it still remains longer 

 than that for sensations of cold and contact. According to 

 Kiesow's latest work, the reaction time to pain sensations is 

 much shortened if sharp-pointed stimuli are used. From this it 

 results that if one and the same cutaneous region is excited 

 simultaneously with cold and hot stimuli, the sensation of cold 



