CH. LI.] 



THE MUSCULAR SENSE. 



703 



Fig. 525. Heat and cold spots. (Waller, 

 after Goldscheider). 



The Sense of Temperature. 



Here again the distribution of acuteness is different ; the tip 

 of the finger is not nearly so sensitive as the forearm or 

 the cheek, to which a washerwoman generally holds her iron 

 when forming a judgment of its 

 temperature. The fraction which 

 represents the discriminative 

 sensibility is approximately ^. 



It has been further shown that 

 there are two kinds of nerve- 

 endings for temperature in the 

 skin which are respectively ex- 

 cited by heat and cold. Thus, 

 if a small metallic pencil kept 

 warm by a stream of water inside 

 it, is moved over the surface, 

 there are some points where the 

 sensation is merely tactile, and 

 at others the pencil will feel 

 uncomfortably hot ; these spots are called heat spots. Cold spots 

 may be similarly mapped out by the use of a cold pencil. The 

 accompanying figure (fig. 525) indicates a small piece of the skin 

 of the thigh with the heat spots horizontally, and the cold spots 

 vertically shaded. 



The Muscular Sense. 



The muscular sense has been much discussed ; some have even 

 denied its existence, and supposed that it is merely a variety of 

 the tactile sense ; when the muscles contract they press upon 

 the skin over them and the joints. No doubt the tactile sense 

 of pressure helps us to know what we are doing with our muscles, 

 but there are two sets of facts which show that the muscular sense 

 proper is different from the tactile sense. One of these is that the 

 muscular sense estimated by the lifting of weights, or by the 

 amount of convergence of the axes of the eyes in looking at objects 

 at different distances, is much more sensitive than the tactile 

 sense of pressure ; the fraction representing the discriminative 

 sensibility being only T ' T instead of or , which is the fraction 

 for the pressure sense. The other set of facts are obtained from 

 the study of disease ; locomotor ataxy is a selective disease ; it 

 may pick out certain sensory tracts and leave others for a time 



