THE SENSES. 161 



come to place unbounded confidence in hat is termed the "evidence of 

 the senses." 



Judgments. We must draw a distinction between mere sensations, 

 and the judgments based, often unconsciously, upon them. Thus, in 

 looking at a near object, we unconsciously estimate its distance, and say 

 it seems to be ten or twelve feet off: but the estimate of its distance is in 

 reality a judgment based on many things besides the appearance of the 

 object itself; among which may be mentioned the number of intervening 

 objects, the number of steps which from past experience we know we 

 must take before we could touch it, and many others. 



Symptoms of Irritation of Nerves of Special Sense. Irritation 

 of the optic nerve, as by cutting it, invariably produces a sensation of 

 light, of the auditory nerve a sensation of some modification of sound. 

 Doubtless these distinct sensations depend not on any specialty in the 

 structure of the nerves of special sense, but on the nature of their con- 

 nections in the sensorium. 



Experiments seem to have proved that none of these nerves possess the 

 faculty of common sensibility. Thus, Magendie observed that when the 

 olfactory nerves, laid bare in a dog, were pricked, no signs of pain were 

 manifested; and other experiments of his seem to show that both the 

 retina and optic nerve are insusceptible of pain. Further, the optic 

 nerve is insusceptible to the stimulus of light when severed from its con- 

 nection with the retina, which alone is adapted to receive luminous im- 

 pressions. 



Sensation of Motion is, like motion itself, of two kinds, progres- 

 sive and vibratory. The faculty of the perception of progressive motion 

 is possessed chiefly by the senses of vision, touch, and taste. Thus an 

 impression is perceived traveling from one part of the retina to another, 

 and the movement of the image is interpreted by the mind as the motion 

 of the object. The same is the case in the sense of touch; so also the 

 movement of a sensation of taste over the surface of the organ of taste, 

 can be recognized. The motion of tremors, or vibrations, is perceived by 

 several senses, but especially by those of hearing and touch. 



Sensations of Chemical Actions. We are made acquainted with 

 chemical actions principally by taste, smell, and touch, and by each of 

 these senses in the mode proper to it. Volatile bodies, disturbing the 

 conditions of the nerves by a chemical action, exert the greatest influ- 

 ence upon the organ of smell; and many matters act on that sense which 

 produce no impression upon the organs of taste and touch, for example, 

 many odorous substances, as the vapor of metals, such as lead, and ths 

 vapor of many minerals. Some volatile substances, however, are per- 

 ceived not only by the sense of smell, but also by the senses of touch and 

 taste. Thus, the vapors of horse-radish and mustard, and acrid suffoca- 

 ting gases, act upon the conjunctiva and the mucous membrane of the 

 VOL. II. 11. 



