CHAP, in.] SIGHT. 863 



but a good many by the third nerve. In the frog the channel 

 is the fourth spinal nerve. Along the spinal cord the dilating 

 influence may be further traced up through the bulb to a centre, 

 which appears to be placed in the floor of the front part of the 

 aqueduct not far from and apparently lateral to the centre for 

 constriction of the pupil. Some authors have supposed that a 

 part of the spinal cord in the lower cervical or upper thoracic 

 region above the origin of the second thoracic nerve has a special 

 share in carrying out the dilating action and hence have called 

 this region the centrum ciliospinale inferius ; but this seems very 

 doubtful. Since, as a rule, a very decided amount of narrowing 

 of the pupils follows upon mere section of the cervical sympa- 

 thetic, we may infer that, unlike the case of the pupil-constrictor 

 mechanism, tonic impulses habitually proceed from the pupil- 

 dilator centre. 



We may trace the path of dilating impulses in the other 

 direction upwards along the cervical sympathetic, not to the 

 sympathetic root of the ciliary ganglion and so to the short 

 ciliary nerves, but to fibres which, passing over the Gasserian 

 ganglion apply themselves to the ophthalmic division of the fifth 

 nerve, and from thence along the nasal branch to the long ciliary 

 nerves, and so to the iris ; while the short ciliary nerves are the 

 channels for pupil-constrictor impulses, the long ciliary nerves 

 are the channels of pupil-dilator impulses. 



541. But while the mode of action of the pupil-constrictor 

 impulses seems clear, since these have simply to throw into con- 

 traction, or increase the contraction of, the fibres of the sphincter, 

 the mode of action of the pupil-dilator impulses is a matter which 

 has been and still is disputed. In the first place, considering 

 how vascular the iris is, it does not seem unreasonable to inter- 

 pret some of the variations in the condition of the pupil as the 

 results of simple vascular turgescence or depletion brought 

 about by vaso-motor action or otherwise. When the blood vessels 

 are dilated and filled they will cause the iris to encroach on the 

 pupil, making the latter small and narrow, and conversely a 

 constricted and emptied condition of the blood vessels would 

 lead to the pupil being large and wide. And indeed slight oscil- 

 lations of the pupil, due to greater or less fulness of the blood 

 vessels, may be observed synchronous with the heart-beat, and 

 others synchronous with the respiratory movements. Hence, 

 remembering how conspicuous a channel for vaso-constrictor 

 impulses is the cervical sympathetic, it seems very natural to 

 suppose that the widening of the pupil which follows upon 

 stimulation of the cervical sympathetic is simply the result of 

 the constriction of the blood vessels of the iris, and conversely 

 that the narrowing of the pupil observed after section of the 

 cervical sympathetic is simply the effect of a greater fulness of 

 the iridic blood vessels resulting from the falling away of the 



