THE SENSES 



971 



Point of tongue - 

 Palmar surface of third 



phalanx of finger 

 Dorsal surface of third 



phalanx of finger 

 Tip of nose - 

 Back - 

 Eyelids 



Skin over sacrum 

 Upper arm - 



Distance at which Two Point'' 

 can l)e distinctly felt, in mm. 



ri 



2'2 



7 



TI'2 

 TI'2 



4'5 

 67-6 



Few of the internal organs are supplied with tactile nerves. The 

 movements of a tapeworm in the intestines are not recognised as 

 tactile sensations, nor the movements of the alimentary canal during 

 digestion, nor the rubbing of one muscle on another during its 

 contraction. 



Pressure is only perceived when it affects two neighbouring areas 

 to a different degree. Thus, the atmospheric pressure, bearing 

 uniformly on the whole surface of the body, causes no sensation ; we 

 are so entirely unconscious of it that it needed the inspiration of 

 genius to discover it, and the persistence of genius to force the dis- 

 covery on the world. When the finger is dipped in a trough of 

 mercury at its own temperature, no sensation is perceived except 

 a feeling of constriction at the surface of the liquid. The perception 

 of light pressure and of the form and size of objects in contact with 

 the skin is believed to be due to the touch-spots. Deep pressure, 

 however, is appreciated, not by the skin, but through sensory end- 

 organs 'in deeper structures probably, e.g., Pacini's corpuscles and 

 the muscle-spindles (Fig. 433, p. 983). 



Sensations of Temperature. When a body colder or hotter 

 than the skin is placed on it, or when heat is in any other way 

 withdrawn from or imparted to the cutaneous tissues with suffi- 

 cient abruptness, a sensation of cold or warmth is experienced. 

 And when two portions of the skin at different temperatures are 

 put in contact, we feel that, relatively to one another, one is 

 warm and the other cold. But it is worthy of remark that it is 

 only difference of temperature (or, perhaps, rather the rate at 

 which heat is being gained or lost by the skin), and not absolute 

 height, which we are able to estimate by our sensations. Thus, 

 a hand which has been working in ice-cold water will feel water 

 at 10 C. as warm ; whereas it would appear cold to a warm hand. 



Blix, Goldscheider, and others have shown that the whole skin 

 is not endowed with the capacity of distinguishing temperature, 

 but that the temperature sensations are confined to minute 

 scattered areas over the cutaneous surface. The great majority 

 of these are ' cold ' spots i.e., respond to stimulation only by a 

 sensation of cold while a smaller number are ' warm ' spots, and 



