490 THE SENSES. 



nerves, the pneumogastric, glosso-pharyngeal, and fifth cerebral 

 nerves, and in its impressions being communicable through 

 many organs. 



Including the sense of touch, the special senses are five in 

 number, the senses of sight, hearing, smell, taste, and touch. 

 The manifestation of each of the first three depends on the 

 existence of a special nerve ; the optic for the sense of sight, 

 the auditory for that of hearing, and the olfactory for that of 

 smell. The sense of taste appears to be a property common 

 to branches of the fifth and of the glosso-pharyngeal nerves. 



The senses, by virtue of the peculiar properties of their sev- 

 eral nerves, make us acquainted with the states of our own 

 body ; and thus indirectly inform us of such qualities and 

 changes of external matter as can give rise to changes in the 

 condition of the nerves. That which through the medium 

 of our senses is actually perceived by the mind is, indeed, 

 merely a property or change of condition of our nerves ; but 

 the mind is accustomed to interpret these modifications in the 

 state of the nerves produced by external influences as proper- 

 ties of the external bodies themselves. This mode of regard- 

 ing sensations is so habitual in the case of the senses which 

 are more rarely affected by internal causes, that it is only on 

 reflection that we perceive it to be erroneous. In the case of 

 the sense of feeling, on the contrary, where many of the pecu- 

 liar sensations of the nerves perceived by the sensorium are 

 excited as frequently by internal as by external causes, we 

 more readily apprehend the truth. For it is easily conceived 

 that the feeling of pain or pleasure, for example, is due to a 

 condition of the nerves, and is not a property of the things 

 which excite it. What is true of these is true of all other 

 sensations ; the mind perceives conditions of the optic, olfac- 

 tory, and other nerves specifically different from that of their 

 state of rest ; these conditions may be excited by the contact 

 of external objects, but they may also be the consequence of 

 internal changes : in the former case the mind, having knowl- 

 edge of the object through either instinct or instruction, rec- 

 ognizes it by the appropriate changes which it produces in the 

 state of the nerves. 



The special susceptibility of the different nerves of sense for 

 certain influences, as of the optic nerve, or rather its centre, 

 for light ; of the auditory nerve, or centre, for vibrations of 

 the air, &c., and so on, is not due entirely to those nerves 

 having each a specific irritability for such influences exclu- 

 sively. For although, in the ordinary events of life, the optic 

 nerve is excited only by the undulations or emanations of which 

 light may consist, the auditory only by vibrations of the air, 



