34 PHYSIOLOGY CHAP. 



predominates and masks the opposite sensation of cold. Thunberg, 

 however, by choosing an appropriate form of stimulus succeeded 

 in producing the two sensations separately, first cold and then 

 heat, which is a strong argument that the nerve-organs for cold 

 lie in a more superficial layer of the skin than those for heat. 



The paradoxical sensation of heat was first observed by 

 Striimpell in anaesthesia produced by freezing, and was described 

 under the name of "perverted thermal sensibility" : Lerda (1905) 

 encountered it in certain small cicatrices of not too recent date ; 

 Michael Sugar (1910) in a patient with syringomyelia and in 

 a few cases of multiple sclerosis; Ponzo (1909) in areas of skin 

 that had been artificially anaesthetised by means of subcutaneous 

 injections of stovaine ; Fontana (1912) in patients suffering with 

 large condyloniata. 



VII. Sensations of pressure, like sensations of heat and cold, 

 are elementary, and cannot be split up into simpler components. 

 Pressure sensations enable us to appreciate the surface contact of 

 external objects independent of their temperature. 



Meissner believed it possible to distinguish sensations of 

 simple contact from pressure sensations, as if they differed funda- 

 mentally. But later observations showed that both are due to 

 deformation of the cutaneous surface, and therefore represent 

 different degrees of a single quality of sensation. When the 

 contact is so light that it produces no pressure on the cutaneous 

 surface, there is no sensation of any kind. 



The functional importance of the sense of contact or pressure 

 lies in the fact that by its means we are able to perceive the 

 slightest mechanical impact upon the surface of our bodies. 

 Meissner's corpuscles and the nerve -rings that surround the 

 sheath of the hairs are homologous organs that come into play 

 during sensations of contact. They are capable of excitation by 

 mechanical agents a thousand times weaker than those necessary 

 for the direct excitation of the peripheral nerves (Tigerstedt). 



We have already pointed out that the heat and cold points 

 do not coincide with the points for contact or pressure. We may 

 now add that the regions of maximal sensibility to thermal 

 stimuli differ from those of sensibility to tactile stimuli. 



An exact comparative determination of tactile or pressure 

 sensibility in the different regions of the skin is very difficult. 

 The pressure exerted by a gas or fluid is certainly the best means 

 of exerting a uniform pressure upon every part of the curved 

 surface of human skin. But we know from experience that such 

 a pressure is not appreciable. We are quite unaware of the 

 pressure exerted by the atmosphere upon the surface of the body 

 as a whole, and of the hydrostatic pressure when the entire 

 body is bathed in water. The physiological effect depends not 

 only on the amount of pressure exerted on the skin, but also on 



