400 AN AMERICAN TENT-BOOK OF PHYSIOLOGY. 



of the anatomical and physiological conditions on which this sensation is 

 dependent. It is said not only that most internal organs possess no def- 

 inite tactile or thermal sensibility, but that, when normal, such irritation as 

 is caused by cutting, burning, and pinching seems to cause no pain; 1 

 let them, however, become inflamed, and their sensitiveness to pain is suf- 

 ficiently acute. The facts of labor-pains, of colic, and other visceral dis- 

 turbances which arc attended by no inflammatory condition show, however, 

 that the factors on which the existence of pain depends are not as yet fully 

 understood. 



The physiological facts on which is based the belief in " common sensa- 

 tion " are indisputable, but the evidence for a special nervous apparatus for 

 such sensibility is based rather on exclusion of known nerve-organs than on 

 positive demonstration. In the category of common sensations have been 

 included also such feelings as " tickling," shivering, hunger, thirst, and sexual 

 sensations. The feeling of fatigue which follows either muscular or mental 

 exertion may be placed in the same group. 



A general feature of common sensations is their subjective character; they 

 are not definitely localized within the body, nor are they projected external to 

 it, as in the case of the " special senses." 



Between the common sensation and its existing cause there is no measurable 

 proportion, as is found, for instance, in the study of the pressure sense. It 

 may be stated that pressure and temperature sensations were within a recent 

 period grouped among common sensations, and future investigations may pos- 

 sibly limit each of the feelings now classed together as "common sensations" 

 to definite anatomical structures. 



When the punctiform distribution of various sensations in the skin is inves- 

 tigated, some points are found in which no other sensation than that of pain 

 can be excited, and it has been thought that such spots mark the place of 

 ending of nerves of common sensibility. 



According to v. Frey, the pain-points are much more numerous than the 

 pressure points, more than 100 falling within a square centimeter of skin, and 

 their nerves are probably more superficial. They require about 1000 times 

 as great an intensity of* stimulus for their excitement as do the pressure- 

 nerves ; they have a long, latent period of stimulation and are inert to rapid 

 changes in the stimulus. This author believes that the free nerve-endings 

 are sense-organs for pain, the end-bulbs for cold, the terminal coils, or net- 

 works, for heat, and the tactile corpuscles for pressure-sensations. 2 



Transferred or " Sympathetic " Pains ; Allochiria. — It has long been a 

 matter of clinical observation that disease seated in certain internal organs is 

 often accompanied by superficial pain and tenderness in widely removed parts 

 of the body; for example, a decayed tooth frequently causes intense pain in 

 the ear ; disease of heart or of aorta may cause pain between the shoulders, 



1 Foster's Physiology, 1891, p. 1420. 



2 Hermann's Jahresbemht ;;. Physiologie, 1897, Bd. v. S. 115; 1896, Bd. iv. S. 113; 1895, 

 Bd. iii. S. 111. 



