860 MOVEMENTS OF THE PUPIL. [Book hi. 



the nasal branch of the ophthalmic division of the fifth nerve. 

 The short ciliary nerves, which are the most numerous, pierce 

 the sclerotic at the hind part of the eyeball and are distributed 

 on the one hand to the blood vessels of the choroid, ciliary pro- 

 cesses and iris, and on the other hand to the ciliary muscle and 

 to the sphincter of the pupil. The less numerous long 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 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 ; this nerve is exceed- 

 ingly complex, and indeed we have reason to consider its 

 ophthalmic division as an independent nerve, which in the 

 course of evolution has become annexed to other nerves to 

 form what we call ' the fifth ' nerve. 



§ 539. We may now make the broad statement, qualifica- 

 tions 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 the observations are con- 

 fined to one eye. When the third nerve is divided, stimulation 

 of the retina or of the 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 ex- 

 treme. 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 stimu- 

 lation 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 condition, con- 

 striction 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 fur- 

 ther dilation. 



