CHAP, in.] SIGHT. 37 



lon^ ciliary nerves, piercing the sclerotic somewhat nearer the 

 front of the eye, are distributed to the muscles of the iris, and 

 probably to the ciliary muscle. 



The third or oculo-motor nerve we may trace back, as we have 

 seen ( 623), to its nucleus in the floor of the aqueduct ; the 

 sympathetic root we may trace back along the cervical sympathetic 

 to the spinal connections of that nerve, on which we have so often- 

 dwelt ; the remarkable ophthalmic division of the fifth nerve we 

 may trace back to the nucleus of the fifth nerve, which we have 

 seen ( 621) to be exceedingly complex, and indeed we have reason 

 to consider this ophthalmic division as an independent nerve, which 

 in the course ol t evolution has become annexed to other nerves 

 to form what we call ' the fifth ' nerve. 



726. We may now make the broad statement, qualifications 

 of which we will consider later on, that constriction of the pupil, 

 brought about by light falling on the retina, is a reflex act, of 

 which the optic is the afferent nerve, the third or oculo-motor the 

 efferent nerve, and the centre some portion of the brain lying in 

 the front part of the floor of the aqueduct at the level of the 

 anterior corpora quadrigemina. This is shewn by the following 

 facts. When the optic nerve is divided, light falling on the retina 

 of that eye no longer causes a constriction of the pupil : we are 

 supposing that tha observations are confined to one eye. When 

 the third nerve is divided, stimulation of the retina or of tlie optic 

 nerve no longer causes constriction ; but direct stimulation of the 

 peripheral portion of the divided third nerve causes constriction 

 of the pupil which may be extreme. If the region of the brain 

 spoken of above as the centre be carefully stimulated, constriction 

 of the pupil will take place even when no light falls on the retina 

 or after the optic nerve has been divided. After destruction of the 

 same region stimulation of the retina is ineffectual in narrowing 

 the pupil. But if the centre and its connections with the optic 

 nerve and third nerve be left intact and in thoroughly sound con- 

 dition, constriction of the pupil will occur as a result of light falling 

 on the retina, though all other parts of the brain be removed. 



It might be imagined that this cerebral centre acted as a 

 tonic centre, whose action was simply increased, not originated, 

 by the stimulation of the retina ; but this is disproved by the 

 fact that if (still dealing with one eye) the optic nerve be divided 

 subsequent section of the third nerve produces no further dilation. 



When the rootlets of the third nerve are separately divided as 

 they leave the brain, it is found that section of those placed more 

 anteriorly interferes with accommodation and constriction of the 

 pupil, while section of the hinder ones affects the ocular muscles. 

 Moreover if the hind part of the floor of the third ventricle and 

 front part of the floor of the aqueduct be carefully explored (in 

 the dog) by means of the interrupted current, the following 

 movements maybe observed in succession as the electrodes are 



