Chap, hi.] SIGHT. 861 



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 may be observed in succession as the elec- 

 trodes are shifted from the front backwards; first movements 

 of accommodation, next constriction of the pupil, and then con- 

 tractions of the ocular muscles. Now in this region lies the 

 elongated nucleus of the third nerve ; 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 reflex pupil-constricting 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. 147 r.5.), 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 ; but visual sensations may be inter- 

 fered with or even abolished, leaving the pupil-constrictor 

 mechanism still active, and on the other hand the afferent limb 

 of the latter may fail, without any impairment of visual sensa- 

 tions. The afferent impulses by means of which light constricts 

 the pupil, seem therefore to take a path of their own ; but the 

 matter is not as yet fully worked out. 



It is desirable to remember one important difference as to the 

 behaviour of the pupil which obtains between man and some of 

 the higher mammals on the one hand, and the lower mammals 

 as w r ell as other vertebrates on the other. In the former, the 

 pupil-constricting nervous mechanisms of the two eyes are not 

 completely independent; there is a functional communion be- 

 tween the two sides, so that when one retina is stimulated both 

 pupils contract, and indeed, in man, as a rule, contract equally. 



