THE SENSES. 549 



of the external bodies themselves. Thus we speak of certain substances 

 as possessing a disagreeable taste and smell; whereas, the fact is, their 

 taste and smell are only disagreeable to us. It is evident, however, that 

 on this habit of referring our sensations to causes outside ourselves (per- 

 ception), depends the reality of the external world to us; and more 

 especially is this the case with the senses of touch and sight. By the co- 

 operation of these two senses, aided by the others, we are enabled gradu- 

 all} r to attain a knowledge of external objects which daily experience 

 confirms, until we come to place unbounded confidence in what 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 the 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 the in- 

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

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



Sensation of Motion is, like motion itself, of two kinds progressive 

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

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

 pression is perceived travelling 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 the 

 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 suffo- 

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

 lungs, exciting through the common sensory nerves, merely modifica- 

 tions of common feeling; and at the same time they excite the sensa- 

 tions of smell and of taste. 



