38 MOVEMENTS OF THE PUPIL [BOOK m. 



shifted from the front backwards ; first movements of accommo- 

 dation, next constriction of the pupil, and then contractions of the 

 ocular muscles. Now in this region lies the elongated nucleus of 

 the third nerve ( 623) ; and it would appear that while the fibres 

 of the third nerve concerned in accommodation arise from the 

 extreme front of the nucleus, those which act upon the pupil start 

 from a succeeding part, the remaining hinder part giving rise to 

 the fibres which govern the ocular muscles. It seems therefore 

 natural to regard the part of the nucleus from which the pupil- 

 constricting fibres spring, as the centre of the refiex pupil-con- 

 stricting mechanism, as the pupil-constrictor centre. 



There is no difficulty as to the connection of the centre with 

 the efferent limb of the reflex chain. The pupil-constrictor fibres 

 pass from the nucleus to the trunk of the third nerve of the same 

 side, and so by the short root to the ciliary ganglion (Fig. 142 r.b.), 

 whence they reach the pupil by the short ciliary nerves ; section 

 of the short ciliary nerves breaks the reflex chain of which we are 

 speaking, and stimulation of them or of their peripheral ends 

 causes narrowing of the pupil. 



But considerable difficulties are met with in determining the 

 connection of the optic fibres, the afferent limb of the chain, with 

 the centre. We should perhaps naturally suppose that the afferent 

 nervous impulses which affected the pupil were the same as, or at 

 least took the same course as those which gave rise to visual 

 sensations, and that there was some connection between the part 

 of the third nucleus serving as the pupil-constrictor centre and 

 one or other or all of the three bodies in which, as we have seen 

 ( 669), the optic fibres end, namely, the lateral corpus geniculatum, 

 the pulvinar, and the anterior corpus quadrigeminum, the connec- 

 tion being perhaps more especially with the latter. But we have 

 no exact or satisfactory knowledge osuch a connection ; and indeed 

 that the connection between the optic fibres and the pupil-con- 

 strictor centre is not furnished by any of these bodies is shewn by 

 the fact that the pupil may still react to light after their removal, 

 though they, or at all events the anterior corpora quadrigemina, 

 are so far connected with the pupil mechanism that interference 

 with them or stimulation of them gives rises to changes in the 

 pupil. The tie, whatever be its nature between the optic fibres 

 and the pupil-constrictor centre, must leave the optic path, if we 

 may use the expression, before these bodies are reached. It has 

 indeed been maintained that the afferent fibres of the optic nerve 

 which affect the pupil do not pass into the optic tract at all, but 

 leave the optic nerve at the decussation, being some of those 

 fibres which pass directly from the optic decussation to the floor of 

 the third ventricle ( 669), and so make their way directly to the 

 nucleus of the third nerve, through the substance of the floor of the 

 third ventricle. But this has been disputed, and other connections 

 have been suggested ; these however we cannot discuss here. 



