40 MOVEMENTS OF THE PUPIL. [BOOK in. 



stimulation of the retina ; in other words a separate complete 

 reflex mechanism exists on each side. If this be so there can he 

 no question of any decussation of either afferent or efferent 

 impulses, and the consensual pupil reaction, the constriction of 

 one pupil caused by retinal stimulation of the other eye, must be 

 carried out by some ties between the two centres of the two eyes. 

 And indeed it has been suggested that the fibres passing from the 

 nucleus of one side to that of the other ( 623) furnish such a tie. 

 If this be the state of things in the dog, it is difficult to believe 

 that the arrangement in other animals is wholly different. We 

 must not however carry on this discussion any further ; but we 

 may perhaps remark that, whichever view we take of the course 

 of the afferent pupil constricting fibres, it is probable that they 

 are not the same as those which subserve visual sensations, that 

 light when it falls on the retina not only excites in certain fibres 

 impulses which give rise to visual sensations, but also excites in 

 other fibres impulses which go to govern the pupil. 



Whatever be the exact nature of the central connections, the 

 existence of such a reflex mechanism is an important fact, since 

 the changes of the pupil which take place in actual life are to 

 a large extent carried out by means of it; a constricted pupil 

 indicates in the majority of instances an activity of the reflex 

 mechanism, and a dilated pupil the absence of or diminution of 

 that activity. In the normal, healthy organism the activity of the 

 mechanism is in the first instance dependent on the amount of 

 light falling on the retina; but even in the normal condition, 

 and still more in an abnormal condition of the organism, other 

 influences may become dominant. The activity of the centre may 

 be exalted or depressed by nervous or other actions ; the retina or 

 optic nerves may be affected by the same amount of light to a 

 degree less than or greater than the normal, and the efferent limb 

 of the chain may be less or more effective. 



727. Besides, however, all the various changes which may 

 thus be induced by playing upon the optic-oculo-motor reflex 

 mechanism, there are other agencies capable of acting on the 

 pupil quite apart from this reflex mechanism. 



If the cervical sympathetic in the neck be divided, all other 

 influences which could possibly affect the pupil being avoided, a 

 constriction of the pupil will be seen to take place ; this however 

 is at times (in animals) not very well marked ; but, whether it 

 be so or no, if the peripheral portion of the nerve (i.e. the upper 

 portion still connected with the head) be stimulated, a well- 

 developed dilation is the result. The cervical sympathetic has, 

 it will be observed, an effect on the pupil, the opposite of that 

 which it exercises on the blood vessels of the head and neck ; 

 when it is divided, the pupil becomes constricted but the blood 

 vessels dilate, and when it is stimulated the pupil is dilated 

 while the blood vessels are constricted. This pupil-dilating 



