590 A TEXTBOOK OF PHYSIOLOGY 



skin is uniformly pressed upon, the sensation of pressure soon 

 disappears; when a finger -is immersed in a bath of mercury, the 

 .sensation of pressure is limited to the area dividing the regions of 

 different pressure. 



Judgments are formed from the sensations received on the supposi- 

 tion that there is no displacement of the body from the normal position. 

 Such a displacement may lead to an erroneous judgment, as exemplified 

 by the experiment of Aristotle. A pea or marble placed between 

 the first and second fingers, held in the normal position, feels one body. 

 If the fingers be crossed, and the pea be so placed as to touch the 

 outer side of both fingers, two peas are felt. So, too, the tip of the 

 nose, touched by the fingers in this position, gives the sensation of 

 two noses, particularly if the eyes be closed at the time. 



The local sign attached to the sensations, when received in the 

 brain, is definite and precise. It is for this reason that a patient's 

 nose shared in a headache after a surgeon had transplanted a piece 

 of skin from his forehead to his nose ! 



The Sensations of Temperature. Sensations of temperature can 

 be evoked from the whole of the skin; the fiont part of the iiares; 

 from the mucous lining of the beginning (mouth, pharynx, oesophagus) 

 and end of the alimentary tract (the region of the anus); from the 

 cornea, conjunctiva, and penis. The spots stimulated by heat and 

 cold are different. Cold spots are more numerous than hot spots, 

 especially over the extremities. They are generally arranged in curved 

 lines. The curves of the hot and cold spots do not, however, coincide. 

 It has been suggested, but the evidence is very- slight, that Krause's 

 end-bulbs and Ruffini's organs are respectively the special structures 

 associated with the appreciation of cold and warmth. 



Two views are held as to what constitutes the adequate stimulus 

 of these sensations. According to one view, the chief factor is 

 the alteration of the end-organ temperature, whatever that may be, 

 a rise of temperature in the end-organ cieating a sensation of warmth, 

 a fall in temperature producing the sensation of cold. Upon this 

 view, it is somewhat difficult to account for the sensation of cold 

 which persists after removal of a cold body. During this time the 

 temperature in the end-organ is rising. For example, after a cold 

 penny (2" C.) has been applied to the forehead for thirty seconds, the 

 sensation of cold may persist for twenty seconds after its removal. 



According to the second view, a skin area gives no temperature 

 sensation when it is at the so-called physiological zero of 

 temperature. This point shifts with the conditions to which the 

 part is exposed. Any alteration of temperature will give rise to a 

 sensation the intensity and character of which depend upon the 

 difference from this point of reference. Thus, if one hand is put 

 into hot and another into cold water, and then both into tepid 

 water, the " hot " hand feels it cold and the " cold '' hand warm. 

 Probably it is the difference between the surface and deep skin 

 temperature (surface and deep sense organs) which gives the intensity 



