THE PROPERTIES OF EACH PART OF THE REFLEX ARC 791 



to finer grades of stimulation. By it we can feel the lightest touch and 

 can discriminate the finest grades of temperature between 26 and 37 C. 

 The power of localization of the stimulus and the ability to discriminate 

 between two points also return with epicritic regeneration. 



In the spinal cord the nerve fibers carrying one kind of sensation 

 are grouped together, in the sense that pain sensations, whether deep or 

 protopathic, run in the same column in the cord. Likewise temperature 

 sensations, whether protopathic or epicritic, run together. 



The Peculiarities of Each of the Separate Sensations 



Temperature. The receptors for temperature are arranged in groups, 

 some being sensitized for heat, others for cold. These groups of receptors 

 are called heat and cold spots. They can be very easily detected on an 



Fig. 204. Thermoesthesiometer. 



area of skin by means of a pointed hollow vessel, through which water is 

 made to flow at a temperature a little below or a little above that of the 

 skin. The instrument is called a thermo-esthesiometer. On a part of the 

 skin where there are no heat and cold spots, the thermo-esthesiometer will 

 elicit no sensation either of heat or of cold. This is charted on an outline 

 drawing of the part as a neutral spot. At other places it will call forth 

 a sensation of heat, indicating the presence' of heat spots, or at others a 

 sensation of cold, indicating the presence of cold spots. It will be noted 

 that certain of the spots are much more reactive than others, and that those 

 of cold are much the more numerous (see Fig. 205). Both heat and cold 

 spots are most frequent at the nipples ; then, in order, come the chest, the 

 nose, the anterior surface of the arm, and the abdomen. They are least 

 marked on the exposed surface of the skin, such as the face, and they are 



