626 THE SENSES. 



sensations peculiar to it. For instance, one uniform internal 

 cause, which may act on all the nerves of the senses in the 

 same manner, is the accumulation of blood in their capil- 

 lary vessels, as in congestion and inflammation. This 

 one cause excites in the retina, while the eyes are closed, 

 the sensations of light and luminous flashes ; in the audi- 

 tory nerve, the sensation of humming and ringing sounds ; 

 in the olfactory nerve, the sense of odours ; 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 aurium" ; 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 sensa- 

 tion of light and colours ; 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 the mechanical impression, or a shock like it. 

 Although, in the cases just referred to, and in all ordi- 

 nary conditions, sensations are derived from peculiar con- 

 ditions of the nerves of sense, whether excited by external 

 or by internal causes, yet the mind may have the same 

 sensations independently 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 : luminous 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 



