THE SENSE OF PAIN. 843 



period preceding its development. The evidence of physiological experiment 

 is against the belief that any irritation of the nerves of .so-called "special 

 senses" can produce pains, but it teaches that this sensation is the result of the 

 excessive or unnatural stimulation of a group of nerves whose function is to 

 give rise to what is indefinitely called " common sensation." By this term is 

 designated that consciousness which we more or less definitely have, at any 

 moment, of the condition and position of the various parts of our bodies. 

 When tactile, temperature, and visual sensations are eliminated, we are still 

 able to designate with considerable accuracy the position of our limbs, and we 

 become aware with extraordinary exactness of any change in that position, 

 indicating the possession of a posture sense. The nerves of common sensation 

 must, then, be continuously active in carrying to the sensorium impulses 

 which, though they do not excite distinct consciousness, probably are of the 

 utmost importance in keeping the nerve-centres informed of the relative posi- 

 tions and physiological condition of the various parts of the organism, and it 

 is not improbable that they are the afferent channels for many reflex acts 

 which tend to preserve the equilibrium of the body. The sudden failure of 

 these sensations in a part of the body would probably be felt as acutely as the 

 silence which succeeds a loud noise to which the ear has become accustomed. 

 Pain is thought to be the result of excessive stimulation of the nerves of com- 

 mon sensation, though it must be admitted that we know next to nothing 

 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 are 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- 



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



