CUTANEOUS SENSATIONS 



543 



itself. In this way a series of ' warm points ' may be mapped out. 

 On now cooling the instrument a few degrees below the 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 

 only appreciated 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, 

 consisting 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 thermo- 

 meter placed within the tube. Having mapped out the warm spots it may be 

 shown that they are excitable by means of mechanical or electrical stimuli 

 and that the sensation produced is the same as if they had been excited by 

 their adequate stimulus, viz. rise or fall of temperature. 





Cold spots. Heat spots. 



FIG. 240. 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.) 



The mapping out 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 appa- 

 ratus 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. 237) giving the topographic distribution of the 

 cold and warm sense-organs on the back 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 



