THE SENSES. 493 



progressive motion is possessed chiefly by the senses of vision, 

 touch, and taste. Thus an impression 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 vibra- 

 tions, is perceived by several senses, but especially by those of 

 hearing and touch. For the sense of hearing, vibrations con- 

 stitute the ordinary stimulus, and so give rise to the perception 

 of sound. By the sense of touch, vibrations are perceived as 

 tremors, ocasionally attended with the general impression of 

 tickling; for instance, when a vibrating body, such as a tuning- 

 fork, is approximated to a very sensible part of the surface, 

 the eye can communicate to the mind the image of a vibrating 

 body, and can distinguish the vibrations when they are very 

 slow ; it may be also that the vibrations are communicated to 

 the optic as to the auditory nerve in such a manner that it re- 

 peats them, or receives their impulses. 



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 influence 

 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 perceived not only by the 

 sense of smell, but also by the senses of touch and taste, pro- 

 vided they are of a nature adapted to disturb chemically the 

 condition of those organs, and in case of the organ of taste, to 

 be dissolved by the fluids covering it. Thus, the vapors of 

 horseradish and mustard, and acrid suffocating gases, act 

 upon the conjunctiva and the mucous membrane of the lungs, 

 exciting through the common sensitive nerves, merely modi- 

 fications of common feeling; and at the same time they excite 

 the sensations of smell and of taste. 



Sensations are referred from their proper seat towards the 

 exterior ; but this is owing, not to anything in the nature of 

 the nerves themselves, but to the accompanying idea derived 

 from experience. For in the perception of sensations, there is 

 a combined action both of the mind and of the nerves of sense; 

 and the mind, by education or experience, has learned to refer 

 the impressions it receives to objects external to the body. 

 Even when it derives impressions from internal causes, it com- 

 monly refers them to external objects. The light perceived 



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