SYSTEM OF THE GREAT SYMPATHETIC. 519 



we pass suddenly from a brilliantly lighted apartment into a dark 

 room, we are unable to distinguish surrounding objects until a 

 certain time has elapsed, and the expansion of the pupil has taken 

 place ; and vision even continues to grow more and more distinct 

 for a considerable period afterward, as the expansion of the pupil 

 becomes more complete. Again, if we cover the eyes of another 

 person with the hand or a folded cloth, and then suddenly expose 

 them to the light, we shall find that the pupil, which is at first 

 dilated, contracts somewhat rapidly to a certain extent, and after- 

 ward continues to diminish in size during several seconds, until the 

 proper equilibrium is fairly established. Furthermore, if we pass 

 suddenly from a dark room into the bright sunshine, we are imme- 

 diately conscious of a painful sensation in the eye, which lasts for 

 a considerable time ; and which results from the inability of the 

 pupil to contract with sufficient rapidity to shut out the excessive 

 amount of light. All such exposures should be made gradually, 

 so that the movements of the iris may keep pace with the varying 

 quantity of stimulus, and so protect the eye from injurious impres- 

 sions. 



The reflex movements of the iris, however, thpugh accomplished 

 through the medium of the ophthalmic ganglion, derive their 

 original stimulus, through the motor root of this ganglion, from 

 the oculo-motorius nerve. For it has been found that if the oculo- 

 motorius nerve be divided between the brain and the eyeball, the 

 pupil becomes immediately dilated, and will no longer contract 

 under the influence of light. The motive power originally derived 

 from the brain is, therefore, in the case of the iris, modified by 

 passing through one of the sympathetic ganglia before it reaches 

 its final destination. 



An extremely interesting fact in this connection is the following. 

 Of the three organs of special sense in the head, viz., the eye, the 

 nose, and the ear, each one is provided with two sets of muscles, 

 superficial and deep, which together regulate the quantity of stimu- 

 lus admitted to the organ, and the mode in which it is received. 

 The superficial set of these muscles is animated by branches of the 

 facial nerve ; the deep-seated or internal set, by filaments from a 

 sympathetic ganglion. 



Thus, the front of the eyeball is protected by the orbicularis and 

 levator palpebrae superioris muscles, which open or close the eye- 

 lids at will, and allow a larger or smaller quantity of light to reach 

 the cornea. These muscles are supplied by the oculo-motorius and 



