PRESSURE AND TOUCH 461 



that the end organs would have a uniform temperature in spite of the varia- 

 tions outside. But this is not the case, for Thunberg has shown that the 

 adaptation is to be observed even after a large quantity of blood has been 

 drawn from the vessels. 



E. H. Weber, to whom we owe the first careful studies on the sensory func- 

 tions of the skin, was of the opinion that the heat spots are stimulated specifically 

 by an increase in the temperature of the skin and the cold spots by a fall. 

 Hering, on the other hand, conceives that the determining factor is not the 

 temperature of the skin but of the thermal apparatus itself. We cannot here 

 enter into a discussion of the merits of these two hypotheses. We may merely 

 mention one phenomenon which can scarcely be explained by either of them. 



It is a common experience that one sometimes feels a sensation of chill on 

 getting into a hot bath. This is because the cold spots of the skin respond to 

 the heat stimulus with their own peculiar sensation. Excitation of these spots 

 by heat occurs only with stimuli of 45 C. and upward (Lehman, v. Frey). The 

 reverse process of stimulating the heat spots by cold gives no reaction. 



The end organs of the temperature nerves share with other nervous end 

 organs a peculiar property, in virtue of which the strength of the excitation 

 aroused depends upon the rapidity of the stimulation, in this case upon the 

 rapidity of the increase or decrease of temperature. The strength of the sensa- 

 tion depends also upon the size of the skin area stimulated : if the whole hand is 

 dipped into water at 37 C. the water feels warmer than water at 40 into which 

 only one finger is dipped. 



If a piece of metal at 2-3 C. be placed in contact with the skin of the 

 brow for, say, thirty seconds and then removed, the sensation of cold on that 

 part of the skin is experienced for some twenty seconds afterwards (E. H. Weber) 

 i. e., the temperature sensation persists after the stimulus is removed. 



A change in the temperature of the skin reduces the excitability -of both 

 kinds of temperature nerve endings. If one hand be held in moderately cold 

 water and the back of the other hand be dipped in the same water, it seems 

 colder to the latter hand. If the skin be overheated then dipped in cold water 

 the water seems warmer than otherwise. 



If hot and cold stimuli be applied simultaneously to the same skin area the 

 sensation of cold appears first. Likewise on stimulation of the cold spots the 

 sensation is sharper and reaches its maximum sooner than the sensation aroused 

 by stimulation of the heat spots. This difference is not observed on stimulation 

 of the temperature spots by electricity. From these facts v. Frey concludes that 

 the heat endings lie in the deeper layers of the skin, the cold endings in the more 

 superficial layers. 



2. PRESSURE AND TOUCH 



By the pressure sense we not only distinguish pressure and contact, but 

 learn also whether the surface of an object is smooth or rough, whether an 

 object is sharp or dull, hard or soft, solid or liquid, etc. Here belong also 

 itching and tickling sensations and the like. 



It is perfectly certain that these different sensations are not all mediated 

 by the true nerves of pressure, but that other afferent nerves play an im- 

 portant part. If for example, we perceive an object to be hard, this sensation 

 of hardness is caused not only by the effect produced on the nerves of pressure, 

 but there is experienced also a sensation of resistance which is evoked by the 



