492 THE SENSES. 



sense of odors ; and in the nerves of feeling, the sensation of 

 pain. In the same way, also, a narcotic substance introduced 

 into the blood, excites in the nerves of each sense peculiar 

 symptoms ; in the optic nerves, the appearance of luminous 

 sparks before the eyes; in the auditory nerves, "tinnitus au- 

 rium ;" and in the common sensitive nerves, the sensation of 

 creeping over the surface. So, also, among external causes, 

 the stimulus of electricity, or the mechanical influence of a 

 blow, concussion, or pressure, excites in the eye the sensation 

 of light and colors ; in the ear, a sense of a loud sound or of 

 ringing ; in the tongue, a saline or acid taste ; and at the other 

 parts of the body, a perception of peculiar jarring or of me- 

 chanical impression, or a shock like it. 



Although, in the cases just referred to, and in all ordinary 

 conditions, sensations are derived from peculiar conditions of 

 the nerves of sense, whether excited by external or by internal 

 causes, yet the mind may have the same sensations independ- 

 ently of changes in the conditions of at least the peripheral 

 portions of the several nerves, and even independently of any 

 connection with the external organs of the senses. The causes 

 of such sensations are seated in the parts of the brain in which 

 the several nerves of sense terminate. Thus pressure on the 

 brain has been observed to cause the sensation of light: lumi- 

 nous spectra may be excited by internal causes after complete 

 amaurosis of the retina ; and Humboldt states, that, in a man 

 who had lost one eye, he produced by means of galvanism, 

 luminous appearances on the blind side. Many of the 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 vi- 

 bratory motion, and capable of being variously changed chem- 

 ically, 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, &c. The information 

 concerning external nature thus obtained by the senses, varies 

 in each sense, having a relation to the peculiar qualities or 

 energies of the nerve. 



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

 progressive and vibratory. The faculty of the perception of 



