954 CUTANEOUS SENSATIONS. 



on the other set of end-organs. It is important to note, however, that, 

 as thus modified, the physiological sensory reaction and the psychological 

 sensory reaction experienced when a skin-surface is changing from a 

 condition of, as regards sensation, " warm " to " less warm," are both 

 totally distinct from the physiological and psychological reactions con- 

 comitant with the transition from " cold " to " more cold," although the 

 physical process initiating both reactions is the same, and although to 

 both, in ordinary parlance, the term " cooling " can be equally correctly 

 applied. And similarly with the comparison between the transitions 

 from " cold " to " less cold," and from " warm " to " more warm." 



Weber 1 suggested, as the internal stimulus of temperature sensations, 

 hypothetical conditions resulting from the fact that bodies undergoing 

 changes in temperature undergo changes in volume. Most substances 

 with rise of temperature expand, and, conversely, as their temperature 

 falls, contract ; and the change in volume is greater in the case of fluids 

 than of solids. The sensorial end-organs of the skin are composed of 

 fluid parts and of more solid parts. It may therefore be " that as tension 

 acting on the parts of the skin in one definite direction generates sensa- 

 tions of touch from certain cutaneous organs, so also distension or 

 collapse, with diminution or increase of tension coincidently in several 

 directions, in certain skin organs generates sensations of cold and warmth." 

 It is in such a manner as this that, Weber 2 suggested, cold stimuli can 

 summate with tactile stimuli for the production of pressure sensations. 



The wall of the aorta was found by Roy 3 to contract in length when 

 warmed. The pressure exerted between a fibrous capsule and fluid contents 

 within it, as the temperature of the whole rose and fell, might alter consider- 

 ably ; or heat may affect the end-organs mediately through pressures generated 

 in the tissues round about them. 



Other stimuli.— Other nervous excitants than thermal are able to 

 excite the apparatus for " cold " and " warmth " sensations in the skin. 

 It is a question how far they excite the end-organs or the nerves. The 

 liminal value for electric stimuli (faradic) applied to the conjunctiva is 

 higher for " cold " sensations than for " pain." i Pressure on the ulnar 

 nerve trunk at the elbow evokes, obscurely, however, sensations of cold 

 or tingling warmth, referred to the ulnar side of the hand more 

 especially. Cold applied to the nerve trunk at the elbow, apart from 

 the sensation of cold produced locally at the place of application, 

 occasions, according to some observers, no sensation of cold 5 in the hand ; 

 in the experience of many, however, although tactual tingling referred 

 to the ulnar side of the hand is the main result, a certain amount of 

 cold sensation referred to the same region is felt also. Such observa- 

 tions are among the most easily demonstrable in illustration of the 

 importance of the end-organ or the efficiency of the adequate stimulus, 

 and for its connection with " specific nerve energy." 



Paradoxical sensation of cold. — A warm surface of a certain temperature 

 applied to a " cold spot " produces a sensation of cold — paradoxical cold 

 sensation. 6 The temperature of the metal applied to elicit the cold sensation 

 has to be about 45° to 50° C. For the skin at large, 45° C. is usually 



1 Loc. cit. 2 Loc. cit., but see also Klug, Arb. a. d. physiol. Anst. zu Leipzig, 1876. 



' s Jour it. Physiol., Cambridge and London, 1882, vol. iii. p. 128. 



4 v. Frey, Err. d. k. sacks. Gesellsch. d. Wisseiisch. zu Leipzig, math.-phys. Classc, loc. cit. 



5 Ludwig, " Lehrbueh der Physiol, d. Menschen,'' Leipzig, 1859, 2te Aufl., 13d. i. 



6 Alf. Lelmiann, " Die Hauptgesetzed. Gefuhlsleben," Leipzig, 1892, S. 35; v. Frey, loc. cit. 



