THE 8EN8E OF PAIN. 399 



the baud and the fingers. The internal mucous surfaces of the body, from 

 the oesophagus to the rectum, inclusive, have no power of discriminating 

 temperature sensations ; a clyster of water cooled to from 7° to 16° C, if not 

 held too long, is only perceived as cold when the water escapes through the 

 skin of the anus. 



The doctrine of specific nerve energy, enunciated by E. H. Weber, was 

 intended to convey the idea elaborated above, that each nerve of special sense, 

 however irritated, gives rise to its own peculiar quality of sensation. But it 

 seems clear that the existence and quality of the sensation are, respectively, 

 properties of the activity, not of the nerve-fibre, but of the peripheral end- 

 organ and the nerve-centres. 



3. Common Sensation and Pain. — The sensations thus far considered 

 have been called special sensations, because each affects the consciousness in 

 quite a different way, and any irritation which excites the sense apparatus 

 provokes a sensation of definite quality and measurable intensity. 



Pain is a sensation which, according to a common but unproved belief, is 

 the result of sufficiently intensifying any of the simple sensations. 



Pains have received various names to distinguish their quality, according to 

 the mode in which experience shows they may have been produced, as cutting, 

 tearing, burning, grinding, etc. One peculiar mark that distinguishes painful 

 sensations is the lack of complete localization. While lesser pains are referred 

 with fair exactness to different parts of the body, and even to those internal 

 parts devoid of tactile sensibility, greater pains radiate and seem diffused over 

 neighboring parts. Pain also differs from special sensation in the long latent 

 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 thai position, 

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

 must, then, be continuously active in carrying to the sensor ium 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 fell 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 



