THE TEMPERATURE-SENSE. 



933 



succession. Gradual variations of a temperature continuously in opera- 

 tion are the more imperfectly recognized the more slowly they take 

 place If two different temperatures are allowed to act at the same 

 time, near each other, the impressions are readily confused, especially 

 if the two positions are close together. 



6. Practice sharpens the temperature-sense; venous congestion 

 of the skin blunts it; ischemia increases its delicacy. The power of 

 differentiation is greater when the application is made to a large cutan- 

 eous surface than when made to a small surface. Rapid variations, 

 further, cause more pronounced sensations than gradual changes of 

 temperature. Fatigue occurs readily. 



7. According to Abrutz strong thermal irritants excite sensations 

 of cold as well as of heat : the feeling of heat is supposed to result from 

 a combination of these two sensations. Direct application of carbon 

 dioxid excites a sensation of warmth ; menthol gives rise to a sensation 

 of coldness and burning. 



FIG. 344. Topography of the Cold-sense and the Heat-sense on the Same Part of the Anterior Surface of the Thigh: 

 a, cold-sense; b, heat-sense. (The dark areas are the highly sensitive, the shaded areas the moderately sensi- 

 tive, the dotted areas the slightly sensitive, and the clear areas the nonsensitive points.) 



Various illusions may occur also in connection with the temperature-sense : 



(1) Occasionally the sensation of heat and cold will alternate in a paradoxical 

 manner: for example if the skin be immersed in water at a temperature of 10 C., 

 a sensation of cold results; if it then be immediately transferred to water at a 

 temperature of 16 C., there is first a sensation of heat, but soon again that of cold. 



(2) The same temperature will be estimated as higher when applied to a large 

 surface of the skin than when applied to a small surface. Thus, the entire hand 

 immersed in water at a temperature of 29.5 feels warmer than when the finger 

 is dipped into water at a temperature of 32 C. Cold weights feel heavier than 

 warm ones. 



Pathological. Sharpening of the tactile sense (hyperpselaphesia) occurs but 

 rarely, although greater sensitiveness for differences in temperature has been 

 observed in places where the epidermis is thinned after the use of vesicants and 

 after vesicular eruptions (zoster) ; likewise in tabetic patients. Sharpening of 

 the spatial sense has been noted also under the two conditions first named and in 

 cases of erysipelas. Brown-Sequard describes as an abnormality of the tactile 



