CUTANEOUS SENSATIONS 



627 



temperature of the surface of the body and then moving it over the surface in 

 the same way, it will be found again that the coolness of the instrument is 

 appreciated only at certain points which can be regarded as ' cold points ' and 

 as containing the nerve-endings by the excitation of which the sensation of 

 cold is produced. If the warm points be pricked out in red -ink and the 

 cold points in blue ink, it will be seen that they do not in any way correspond. 



A convenient instrument for this purpose is the one invented by Miescher, con- 

 sisting of two tubes cemented together and communicating at a small flattened extremity, 

 which is applied to the surface of the skin ; through the tubes water can be led at any 

 desired temperature, which is read off by a thermometer placed within the tube. Having 

 mapped out the warm spots it may be shown that they are excitable by means of mechani- 

 cal or electrical stimuli and that the sensation produced is the same as if they had been 

 excited by their adequate stimulus, viz. a rise of temperature. 



II 



Cold spots. Heat spots. 



FIG. 314. Heat and cold spots on part of palm of right hand. 



The sensitive points are shaded, the black being more sensitive than the lined, 

 and these than the dotted parts. The unshaded areas correspond to those parts 

 where no special sensation was evoked. (GOLDSCHEIDEB. ) 



EXACT LOCATION of the spots is rendered difficult by the irradiation 

 of the sensation produced, so that it is difficult to refer the sensation of 

 warmth or cold definitely to the point stimulated. An investigation of the 

 topography of these warm and cold spots shows that the apparatus for the 

 appreciation of cold is much more extensively distributed over the body than 

 that for the appreciation of warmth, as is evidenced from the diagram 

 (Fig. 314) giving the topographic distribution of the cold and warm sense- 

 organs on the palm of the hand. The temperature sense is best marked in 

 the following regions of the body : the nipples, chest, nose, the anterior 

 surface of the upper arm and the anterior surface of the fore-arm, and the 

 surface of the abdomen. It is much less marked on the exposed parts of the 

 body, such as the face and hands, and is but slight in the mucous membranes. 

 Thus it is possible to drink hot fluid, such as tea, at a temperature which 

 would be painful to the hand, and still more to any other part of the body. 

 The scalp is also very insensitive to changes of temperature. The acuteness 

 of the temperature sense varies considerably with the condition of the skin 

 and with the previous stimulation of the sense organs. The sense is most 

 acute at about ordinary skin temperature, i.e. between 27 and 32 C. At 

 this temperature the skin can appreciate a difference of -J- C. When the 



