494 THE SENSES. 



in congestion of the retina seems external to the body ; the 

 ringing of the ears in disease is felt as if the sound came from 

 some distance ; the mind referring it to the outer world from 

 which it is in the habit of receiving the like impression. 



Moreover, the mind not only perceives the sensations, and 

 interprets them according to ideas previously obtained, but it 

 has a direct influence upon them, imparting to them intensity 

 by its faculty of attention. Without simultaneous attention, 

 all sensations are only obscurely, if at all, perceived. If the 

 mind be torpid in indolence, or if the attention be withdrawn 

 from the nerves of sense in intellectual contemplation, deep 

 speculations, or an intense passion, the sensations of the nerves 

 make no impression upon the mind ; they are not perceived, 

 that is to say, they are not communicated to the conscious 

 " self," or with so little intensity, that the mind is unable to 

 retain the impression, or only recollects it some time after, 

 when it is freed from the preponderating influence of the idea 

 which had occupied it. 



This power of attention to the sensations derived from a 

 single organ, may also be exercised in a single portion of a 

 sentient organ, and thus enable one to discern the detail of 

 what would otherwise be a single sensation. For example, by 

 well-directed attention, one can distinguish each of the many 

 tones simultaneously emitted by an orchestra, and can even 

 follow the weaker tones of one instrument apart from the other 

 sounds, of which the impressions being not attended to are less 

 vividly perceived. So, also if one endeavors to direct atten- 

 tion to the whole field of vision at the same time, nothing is 

 seen distinctly ; but when the attention is directed first to this, 

 then to that part, and analyzes the detail of the sensation, the 

 part to which the mind is directed is perceived with more dis- 

 tinctness than the rest of the same sensation. 



THE SENSE OF SMELL. 



The sense of smell ordinarily requires, for its excitement to 

 a state of activity, the action of external matters, which action 

 produces certain changes in the olfactory nerve ; and this 

 nerve is susceptible of an infinite variety of states dependent 

 on the nature of the external stimulus. 



The first condition essential to the sense of smell is the exis- 

 tence of a special nerve, the changes in whose condition are 

 perceived as sensations of odor ; for no other nerve is capable 

 of these sensations, even though acted on by the same causes. 

 The same substance which excites the sensation of smell in the 

 olfactory nerves may cause another peculiar sensation through 

 the nerves of taste, and may produce an irritating and burn- 



