GENEEAL AND SPECIAL SENSIBILITY. 839 



to the eye or ear after a while fail to impress them. So, also, odors 

 may cease to be recognized. 



This point, however, must not be misunderstood. If continued 

 attention be directed to sensations, instead of being blunted they are 

 exalted. Of this we have numerous examples in the capability of the 

 senses to attain a high degree of acuteness of perception from education. 



When an impression has been made upon a nerve of special sensi- 

 bility, or even upon one of general sensibility, that impression always 

 remains a certain length of time after the irritating cause has been 

 removed ; thus, when an ignited stick is rapidly moved before the eye 

 the impression made upon the retina in each successive position of the 

 burning point remains sufficiently long to appear continuous with that 

 made in the next situation, and thus the appearance of a line of light 

 is obtained. So in the case of the ear ; it is well recognized that 

 musical tones are produced by a regular succession of vibrations. When 

 these vibrations succeed each other more frequently than sixteen times 

 in a second they give rise to a continuous tone. When they occur less 

 frequently than sixteen times in a second there is produced a succession 

 of impressions, each one terminating before the other begins. No nerve 

 of special sense can take upon itself the function of airy other ; thus the 

 auditory nerve is incapable of transmitting visual impressions, nor can 

 the olfactory nerve serve for audition. In the case of the nerves of 

 general sensibility, the incapability of interchange of function cannot 

 be so positively denied, since we know that a motor nerve may so unite 

 with a sensory nerve as to conduct afferent impressions ; or in the case 

 of the sense of taste, which is one of the lowest of the special senses, 

 we shall find that there other than the special nerves of taste may, 

 perhaps, serve for conducting gustatory impressions. 



While the nerves of special sense are especially adapted for receiving 

 certain impressions, they may yet be thrown into a condition of irritability 

 by various stimuli, but each of these stimuli will produce the impression 

 characteristic of the nerve over which it passes. 



Sensations have been divided into two classes, external and internal, 

 or objective and subjective. External impressions are those which arise 

 from impressions made upon the external surface of the body. By 

 internal impressions are meant those which arise from impressions made 

 upon the internal recesses of the body. All sensations, however, originate 

 and depend upon changes taking place in the gray matter of the brain 

 itself, and, as a consequence, there is no sensation (which in general 

 terms is produced by an impression made upon the peripheral termi- 

 nation of the nerve) which may not to a certain degree be produced 

 by an impression made upon the nerve in its course or at the 

 point in the sensorium where the nerve terminates. To this latter 



