CHAP, in.] SIGHT. 1179 



influence of the cervical sympathetic may, as in the case of 

 the vaso-constrictor action of the same nerve, be traced back- 

 wards down the neck to the upper thoracic ganglion, and thence to 

 the spinal cord along the ramus communicans and anterior root of 

 the second thoracic nerve, in the case of the dog, the monkey and 

 man ; in the frog it is the fourth spinal nerve. From thence 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 cer- 

 vical sympathetic, 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. 



728. 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. We have already ( 724) urged 

 that the widening of the pupil cannot be simply the result of vaso- 

 constrictor action on the blood vessels of the iris ; and it is stated 

 that the long ciliary nerves which act as pupil-dilators carry no 

 vaso-constrictor impulses ; it is said that stimulation of the long 

 ciliary nerves while it widens the pupil produces no vaso-motor 

 effects, and after the division of the long ciliary nerves stimulation 

 of the cervical sympathetic, while it produces vaso-constriction in 

 the eye as in other parts of the head and face, gives rise to no 

 widening of the pupil. The impulses then which pass along the 

 long ciliary nerves must affect in some manner or other the 

 muscles of the iris other than those of its blood vessels. 



Two views are held as to what that manner of action exactly 

 is. Those who regard the radially disposed tissue lying im- 

 mediately in front of the hind pigment layer of the iris as a 

 muscular layer of radiating plain muscular fibres, maintain 



