BRAIN AND NERVOUS SYSTEM. 335 



767. The nerves of special sense are subject to the same 

 Jaw as the sensory nerves ; they receive natural impressions 

 at their extremities, but not in their course. They can con- 

 vey to the brain, not common feeling of pleasure or pain, but 

 such impressions as are made on their outer terminations, 

 which are expanded in the several organs of sense. The 

 optic nerve conveys the impression of light, the auditory con- 

 veys sound, the gustatory conveys taste, and the olfactory 

 nerve conveys the impression of odors. When these nerves 

 are irritated, or touched, or diseased, they still excite similar 

 sensations. If we close the eye and press the ball upon the 

 optic nerve, an impression is made upon the brain similar to 

 that caused by light. If we strike suddenly upon the eye, or 

 even the temple, so as to jar this nerve, the brain sees flashes 

 of light. Dr. Howe has often tried these experiments with 

 the blind, both with those who were born in this condition, 

 and had never seen light, and with those who became blind 

 after birth, and they all saw flashes and stars. 



768. When we receive a blow on the side of the head, so 

 as to jar the auditory nerve, or when the ear is diseased, we 

 hear a ringing in the ear. In some states of disease, men 

 complain of bad taste on the tongue. In vain they wash and 

 purify the mouth still the offensive taste remains ; for it is 

 not an impression made upon the tongue and carried thence 

 to the brain, but the impression of some disturbance or 

 derangement of the trunk of the nerve excites a disagreeable 

 sensation in the brain, which refers it back, not to the spot 

 which is disturbed, but to the termination in the tongue. 



769. Whatever excites any nerve in its c< urse will pro- 

 duce an effect upon the brain similar to that which is pro- 

 duced by impressions made upon its terrain itions, and the 

 sensations will be referred back to the ends c f the nerve as 

 their seats. An electric shock, if passed through the nerve 

 of the ear, will give the sensation of sound ; and if through 

 the nerve of the eye, the sensation of light. If we apply a 

 piece of zinc and copper to the upper and lower surface cf 

 the tongue, and let their edges touch each other, they excite 

 a sense of unpleasant taste. 



