THE SYMPATHETIC SYSTEM. 587 



motor root from the oculomotorius. The reflex action, by which the 

 pupil contracts under the influence of light and expands under its 

 diminution, takes place, accordingly, through this ganglion. The impres- 

 sion conveyed by the optic nerve to the tubercula quadrigemina, and 

 reflected outward by the fibres of the oculomotorius, is not transmitted 

 directly by the last named nerve to the iris ; but passes first to the 

 ophthalmic ganglion, and is thence conveyed to its destination by the 

 ciliary nerves. 



The reflex movements of the iris exhibit consequently a somewhat 

 sluggish character, which indicates the intervention of the sympathetic 

 system. The changes in the size of the pupil do not take place instan- 

 taneously with the variation in the amount of light, but require an 

 appreciable interval of time. If we suddenly pass from a light 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 distinct for a consider- 

 able period afterward, as the expansion of the pupil becomes more com- 

 plete. If we cover the eyes of another person with the hand or a folded 

 oloth, and then suddenly expose them to the light, we can see that the 

 pupil, which is at first dilated, contracts somewhat rapidly to a certain 

 extent, and afterward continues to diminish in size for several seconds, 

 until its equilibrium is fairly established. Furthermore, if we pass sud- 

 denly from a dark room into bright sunshine, we are immediately con- 

 scious of a painful impression in the eye, which results from the inability 

 of the pupil to contract with sufficient rapidity. All such exposures 

 should therefore be made gradually, in order that the movements of the 

 iris may keep pace with the varying quantity of stimulus, and thus 

 protect the eye from injurious impressions. 



The reflex movements of the iris, though accomplished through the 

 medium of the ophthalmic ganglion, derive their original stimulus, 

 through the motor root of this ganglion, from the oculomotorius nerve. 

 For if the oculomotorius nerve be divided between the brain and the 

 eyeball, the pupil becomes sensibly dilated, and loses in great measure 

 its power of contracting under the influence of light. The motive power, 

 originally derived from the brain, is, therefore, modified by passing 

 through the ophthalmic ganglion before reaching its destination in the 

 iris. 



Three organs of special sense, the eye, the nose, and the ear, are 

 each provided with two sets of muscles, superficial and deep, which 

 regulate the quantity of stimulus admitted to the organ and the mode 

 in which it is received. The superficial set 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 palpebrse superioris muscles, which open or close the eyelids at 

 will, and allow a larger or smaller quantity of light to reach the cornea. 

 These muscles are supplied by the oculomotorius and facial nerves, arid 



