i CUTANEOUS SENSIBILITY 31 



precedes that of heat. Further, the excitation of any spot by 

 cold produces a more lively sensation, that reaches its maximum 

 more rapidly than excitation of the same spot by heat. Accord- 

 ing to v. Frey this difference is not apparent on exciting the two 

 thermal spots by electrical stimuli. From this he concluded that 

 the nerve-organs of the warm spots lie in the deeper layers of thei / 

 skin, and those of the cold spots in the more superficial layers. ? 



The physical properties of the thermal agents, again, have an 

 influence on the effects of excitation. Stimuli may consist of 

 solid, liquid, or gaseous bodies, and may act by conducting heat or 

 by irradiation ; they may be good or bad thermal conductors ; 

 their thermal capacity may be large or small ; lastly, they may 

 have a smooth or a rough surface. 



Thermal sensations are stronger according as the stimulating 

 body is a good conductor of heat. Water at 25 C. is a stronger 

 stimulus of cold than oil, and less strong than mercury at the 

 same temperature. It is possible to arrange a graduated series of 

 bodies with different thermal conductivities, but all of the same 

 temperature, by which a series of thermal sensations of gradually 

 increasing strength can be excited. This, however, applies only to 

 intensity of sensation as evoked by the initial contact. With 

 prolonged contact new relations are set up, due to variations in 

 the thermal exchanges between the cutaneous surface and the 

 external agent, so that a first impression of cold may be translated 

 into a sensation of warmth. For instance, on dressing, or lying 

 down in bed undressed, the first sensation is one of cold, followed 

 quickly by the opposite sensation of warmth, which may be less or 

 greater according to the nature and thickness of the clothing or 

 bed-covering. 



Any body that serves as a thermal stimulus must, besides its 

 power of conducting heat, also possess a certain minimal thermal 

 equation in order to produce a sensation ; the latter within certain 

 limits may increase in intensity with an increasing thermal 

 equation of the stimulating body. Thunberg has shown that 

 various degrees of thermal excitation can be evoked in the skin by 

 contact with bodies that have the same temperature but different 

 thermal properties, for instance a series of silver or copper plates 

 of various thicknesses. By means of these plates it is easy to 

 determine the minimal degree of heat required to evoke a thermal 

 sensation. 



The importance of the smoothness or roughness of the surface 

 of the body that is used as a thermal stimulus is easily under- 

 stood, seeing that the conduction of heat, and hence the efficacy 

 of stimulation, varies according as the points of contact between 

 the skin and the conducting body are few or many. 



The essential conditions for the production of sensations of 

 heat or cold must consist in the thermal changes that take place 



