6z8 THE SENSES. 



various morbid sensations attending diseases of the brain, 

 the vision of spectra, and the like, are of the same kind. 



Again, although the immediate objects of the perception 

 of our senses are merely particular states induced in the 

 nerves, and felt as sensations, yet, inasmuch as the nerves 

 of the senses are material bodies, and therefore participate 

 in the properties of matter generally, occupying space, 

 being susceptible of vibratory motion, and capable of being 

 variously changed chemically, as well as by the action of 

 heat and electricity, they make known to the mind, by 

 virtue of the different changes thus produced in them by 

 external causes, not merely their own condition, but also 

 some of the different properties and changes of condition 

 of external bodies ; as, e.g., progressive and tremulous 

 motion, chemical change, etc. The information concerning 

 external nature thus obtained by the senses, varies in each 

 sense, having a relation to the peculiar qualities or ener- 

 gies of the nerve. 



The sensation of motion is, like motion itself, of two kinds, 

 progressive and vibratory. The faculty of the percep- 

 tion of 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 

 recognised. The motion of tremors, or vibrations, is 

 perceived by several senses, but especially by those of 

 hearing and touch. For the sense of hearing, vibrations 

 constitute the ordinary stimulus, and so give rise to the 

 perception of sound. By the sense of touch, vibrations 

 are perceived as tremors, occasionally 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 communi- 



