THE SENSES. G63 



gradually increasing. When they reach the lips they seem to be consid- 

 erably further apart than on the cheek. Thus, too, our estimate of the 

 size of a cavity in a tooth is usually exaggerated when based upon sensa- 

 tion derived from the tongue alone. Another curious illusion may here 

 be mentioned. If we close the eyes, and place a small marble or pea 

 between the crossed fore and middle fingers, we seem to be touching two 

 marbles. This illusion is due to an error of judgment. The marble is 

 touched by two surfaces which, under ordinary circumstances, could 

 only be touched by two separate marbles, hence the mind, taking no 

 cognizance of the fact that the fingers are crossed, forms the conclusion 

 that two sensations are due to two marbles. 



Temperature. The whole surface of the body is more or less sen- 

 sitive to differences of temperature. The sensation of heat is distinct 

 from that of touch : and it would seem reasonable to suppose that there 

 are special nerves and nerve-endings for temperature. At any rate the 

 power of discriminating temperature may remain unimpaired when the 

 sense of touch is temporarily in abeyance. Thus if the ulnar nerve be 

 compressed at the elbow till the sense of touch is very much dulled in 

 the fingers which it supplies, the sense of temperature remains quite 

 unaffected. 



The sensations of heat and cold are often exceedingly fallacious, and 

 in many cases are no guide at all to the absolute temperature as indi- 

 cated by a thermometer. All that we can with safety infer from our 

 sensations of temperature, is that a given object is warmer or cooler 

 than the skin. Thus the temperature of our skin is the standard ; and 

 as this varies from hour to hour according to the activity of the cutane- 

 ous circulation, our estimate of the absolute temperature of any body 

 must necessarily vary too. If we put the left hand into water at 5 C. 

 (40 F.) and the right into water at 45 0. (110 F.), and then immerse 

 both in water at 27 C. (80 F.), it will feel warm to the left hand but 

 cool to the right. Again, a piece of metal which has really the same 

 temperature as a given piece of wood will feel much colder, since it con- 

 ducts away the heat much more rapidly. For the same reason air in 

 motion feels very much cooler than air of the same temperature at rest. 



In some cases we are able to form a fairly accurate estimate of abso- 

 lute temperature. Thus, by plunging the elbow into a bath, a practised 

 bath-attendant can tell the temperature sometimes within half a degree 

 centigrade. 



The temperatures which can be readily discriminated are between 

 10-45 C. (50-115 F.); very low and very high temperatures alike 

 produce a burning sensation. A temperature appears higher according 

 to the extent of cutaneous surface exposed to it. Thus, water of a tem- 

 perature which can be readily borne by the hand, is quite intolerable if 



