THE AFFERENT PART OF THE NERVOUS SYSTEM 77 



usual field. But the more generally accepted view is 

 that pain is a sensation for which a particular nervous 

 mechanism is requisite and that the nerves concerned can 

 give rise only to painful sensory phenomena. 



Experimental study supports the a priori deductions 

 in regard to the multiple character of the receptor equip- 

 ment of the skin. If one applies to the arm a heated rod 

 of glass, one's first impression is that sensations of press- 

 ure and temperature perhaps of pain also are produced 

 by stimulation of a single spot. But if the trial is made 

 more carefully, employing a slender instrument for the 

 test and avoiding deformation of the skin, one brings to 

 light what is called the "punctiform" distribution of 

 sensibility. By this expression is meant the existence of 

 particular areas of very small size from which particular 

 sensations can be aroused, while others cannot. 



It is possible to mark off a chosen region on any part of 

 the surface of the body and to determine within its 

 boundaries the points which respond appropriately to 

 four forms of stimulation by the four kinds of sensation 

 enumerated above. Four sets of points can then be indi- 

 cated in color, and if the cutaneous map is preserved for a 

 time the points are found to be permanent. They must be 

 situated just over the endings of four distinct though more 

 or less commingled sets of nerve-fibers. It is now easy to 

 see how more than one kind of sensation may seem to be 

 produced when the skin is touched. If the object which 

 makes the contact is rather blunt, it is certain to affect 

 one or more pressure-points, while it will either give or 

 take heat to such an extent as to stimulate neighboring 

 organs which are sensitive to temperature changes. 

 The skin is like a mosaic so finely subdivided that a close 

 examination is necessary to resolve it into its constituent 

 parts. 



We must expect that stimulation of a selected ending 

 in the skin by unusual means will give no sensation but the 

 one proper to that fiber. It is, in fact, rather difficult to 

 excite a pressure-point by forms of stimulation other than 



