528 



VISION 



A B 



FIG. 222. The iris of a cat. A, at rest; 

 B, on stimulating it at the upper 

 right-hand side (Langley). 



nerves to the sphincter pupillse. 



Constriction of the pupil is caused by contraction of a circular muscle, 

 composed in most animals of smooth muscle fibers and known as the sphinc- 

 ter of the pupil; dilatation, by smooth radial fibers known collectively as the 

 dilator of the pupil. The existence of an independent dilator was conclu- 

 sively proved several years ago by the physi- 

 ological experiments of Langley and An- 

 derson; it has recently been demonstrated 

 anatomically as well. 



The muscles of the iris receive their 

 nerves by both cerebral and sympathetic 

 pathways. The constrictor fibers are found 

 in the oculo motor. From this nerve they 

 pass over to the ciliary ganglion,, connect 

 there with nerve cells (Langendorff), and 

 continue thence through the short ciliary 

 Stimulation of a single one of the short 

 ciliary nerves causes only partial contraction of the sphincter, so that the 

 pupil takes an irregular form. It is stated that the oculo motor at the same 

 time inhibits the dilator of the pupil. 



The dilator fibers of the pupil come from the sympathetic. They pass out 

 of the spinal cord by the anterior roots of the seventh to the eighth cervical and 

 the first to the second thoracic spinal nerves, go to the first thoracic ganglion, 

 then through the anterior arm of the anmilus of Vieussens to the inferior 

 cervical ganglion, and from this by way of the trunk of the cervical sympathetic 

 to the superior cervical ganglion. From the superior cervical ganglion, the 

 fibers pass to the Gasserian ganglion, follow the trigeminal and traverse the 

 long ciliary nerves, without connecting with the ciliary ganglion, to the iris. 

 Stimulation of the sympathetic is said to cause inhibition of the sphincter as 

 well as excitation of the dilator (Reid). 



Both constrictor and dilator fibers of the pupil are in a state of tonic 

 excitation: when the cervical sympathetic is cut, the pupil constricts; when 

 the oculo motor is cut, it dilates. 



The following experiments by Langley and Anderson show that the dila- 

 tation of the pupil is not merely a matter of inhibition on the part of the 

 sphincter pupillse. When the sclerotic was stimulated locally with the induc- 

 tion current, a short local dilatation of the pupil (Fig. 222) was obtained. 

 Were the dilatation due solely to inhibition of the sphincter, 

 the movement would have been uniform all around the pupil. 

 Again, when a sector of the iris is isolated except at its ciliary 

 attachment by two radial cuts (Fig. 223), this sector shortens 

 both on direct stimulation and on stimulation of the cervical 

 sympathetic. 



FIG. 223. After 



Changes in the diameter of the pupil under normal cir- Langley. 



cumstances are for the most part produced reflexly. The most 

 important of these reflexes is mediated by the optic nerve. Constriction begins 

 within 0.4-0.5 second and reaches its maximum in about 0.1 second thereafter 

 (Listing) . It requires but an instantaneous flash of light to call out the reflex 

 constriction (v. Vintschgau). 



In all animals in which only part of the optic nerve fibers cross at the 



