PHYSIOLOGY. 33 



" This may be illustrated. Suppose we had a scale mark- 

 ing the different degrees of force from 1 to 100, as in the case of 

 heat, I might say that till the degree of force rises to 20, no 

 sensation is produced ; and that at the degree of 100 the organ 

 is destroyed ; and at 80 the force produces the undetermined 

 sensation of pain ; and, in this way, our sensations are limited 

 between the force of 80 and that of 20 of such a scale. When 

 I say that a high force of impression sometimes entirely destroys 

 the organ, I illustrate it by an intense sound breaking the mem- 

 brane of the ear, and by a very powerful acrimony, or a great de- 

 gree of heat or cold, destroying the mixture or texture of our solids. 

 There is, however, a degree of force which destroys sensibility 

 without destroying the mixture or texture of the organ. We can- 

 not perceive in what manner it has acted upon the texture or mix- 

 ture, but we are sensible that, when the stupor is recoverable, it 

 has merely changed that state of the organization upon which sen- 

 sation depends. Thus, a certain intensity of sound is not sufficient 

 to destroy the organ of the ear, but only to give a stupor for some 

 time ; and thus the eye may, by the action of a strong light, be 

 rendered insensible to light, or to any object. I remember a 

 country schoolmaster who had been a menial servant to a profes- 

 sor of mathematics, and who wishing to maintain some reputation 

 as an astronomer, would view an eclipse with a tube without 

 glasses : he was struck blind for three months. In like man- 

 ner, a certain percussion on the elbow will give a stupor to the 

 whole arm ; and it is in this way that we explain the effects of 

 the stroke of the torpedo, and those of electricity. A concussion 

 of the brain also produces an absolute insensibility for a time ; 

 but it soon appears that it has not destroyed the organization." 

 XLIII. Within these limits, however, our sensations are not 

 exactly correspondent to the force of impression, nor do they 

 make any exact estimate of that force. Usually, sensation is 

 relative to the change that is produced in the nervous system ; 

 and a sensation proves strong or weak only as it is stronger or 

 weaker than that which had immediately preceded it, or than that 

 degree of force to which the nerves had been immediately be- 

 fore accustomed. For this reason too, the limits (XLII.) are 

 very variable. " Suppose the impression to have a force of 50, 

 it is possible that, in different circumstances, it may appear 

 VOL. i. c 



