956 CUTANEOUS SENSATIONS. 



" burn." For a similar reason, a hot glass or a live coal can be touched 

 with impunity to pain if the contact last but a very short time. (3) It 

 depends to a remarkable degree on the size of the area of application of 

 the stimulus. This last point entails separate description. 



As stated above (under Touch), a stimulus applied to a sensifacient 

 surface, like retina or skin, has an influence on sense not only in virtue 

 of its intensity, but also in virtue of its extension in space as well 

 as in time. So that, cceteris paribus, a stimulus affecting a larger area 

 evokes more sensory reaction than one affecting a smaller area. In 

 accordance with the overlapping arrangement of the mechanisms of 

 the nervous system, a stimulus simply in virtue of greater extent of 

 application to the sensifacient surface evokes, at least in some cases, 

 greater intensity of sensation. This is markedly exemplified in the 

 cutaneous senses of " cold " and " warmth." 



If one finger of one hand and the whole other hand are dipped simul- 

 taneously into the same cold or warm fluid, the sensation of cold or warmth is 

 not equally intense in the two places ; it is stronger in the larger surface. 

 Weber wrote: 1 "This greater strength of impression, which results from the 

 same impression acting upon a greater number of sensory points, is easily 

 confused with that strength of impression which, under other circumstances, 

 results from the fluid having a higher or lower temperature. Cold water, 

 therefore, feels colder to the whole hand, warm water warmer, than to a single 

 finger, in spite of the subject nevertheless knowing that the temperature of the 

 water is the same. If one dips a finger of one hand and the whole other hand 

 into water the temperature of which one is ignorant about, one is deceived. 

 Water at 29 0, 5 R. is, if the whole hand is dipped in it, thought to be warmer 

 than water at 32° R. 2 into which one dips a single finger. Similarly, one is 

 deceived in case of water at 17° R. and 19° R. 



It is an important observation of Weber's, that the reinforcing influence 

 which one thermal stimulus, e.g. of cold, has upon the intensity of another 

 (of the same species) simultaneously applied, is greater when the places of 

 application of the two stimuli lie near than when they are far apart. For 

 instance, the difference between the temperatures of two equal surfaces can be 

 distinguished with greater nicety when the surfaces are large than when small, 

 but when the two smaller surfaces are applied not far apart from one another, 

 the discrimination is more exact than between the larger surfaces if the latter 

 be applied a considerable distance from each other. 



So is it with the pain concomitant of cold and warmth sensations occasioned 

 by thermal stimuli of excessive temperature. The bath comfortably hot to 

 the finger of the nurse, may be very uncomfortably hot to the immersed child 

 or patient. The finger can be kept for any length of time in water at 48° C, 

 but the whole hand held in, soon feels prolonged immersion painful. On the 

 view taken by Goldscheider, therefore, in pain sense, as in thermal sense, mere 

 areal increase of stimulation induces higher intensity of sensory reaction. 



Besides the above factors determining the temperatures at which stimuli 

 cause pain, as well as act on thermal sense, are of course others such as 

 specific heat and the thermal conductivity of the object. 



The objective temperature of the sensifacient thermal organs 

 of an area of skin, in which we are conscious of neither cold 

 nor warmth, is called the physiological zero temperature. It is not 

 a constant objective temperature ; it differs in different areas of skin, 



1 Loc. cit. 



2 Gad says water at 27° C. to the whole hand feels warmer than water at' 40° C. to the 

 finger. This overstates the effect in my own experience, hut my sensation gives figures 

 nearer to Gad than to Weher. 



