THE ENCEPHALIC OR CRANIAL NERVES 



633 



contraction of the radiating fibers of the iris, which collectively constitute 

 the dilatator pupillcB muscles. With the entrance of light into the eye, 

 one or both, the pupils diminishes in size, in consequence of the contraction 

 of the sphincter pupillcB muscles. Observation shows that the diminution 

 in size of the pupil is not confined to the corresponding eye, but is mani- 

 fested in the eye of the opposite side as well. In other words the con- 

 traction of the sphincter pupillae is not only direct but consensual. 



The action of the sphincter pupillae muscle is, therefore, a reflex action 

 and involves the usual mechanism, viz. : A receptive surface, the retina ; 

 afferent nerves, the pupil- 

 lary fibers in the optic 

 nerve and tract; a receiv- 

 ing intercalated mechan- 

 ism; an emissive center, 

 the sphincter nucleus of 

 the motor oculi center; 

 efferent nerves, including 

 fibers in the trunk of the 

 motor oculi and in the 

 ciliary nerves; and a re- 

 sponsive organ, the muscle. 

 (See Fig. 264.) That this 

 is the mechanism involved 

 .is shown by the fact that 

 when any portion of it is 

 destroyed, the reflex con- 

 tractions of the sphincter 

 are impaired or abolished. 



As stated in a preceding 

 paragraph the central ter- 

 mination of the afferent 

 pupillary fibers concerned 

 in this reflex is not posi- 

 tively known. No one has 

 as yet succeeded in tracing 

 these fibers directly to the 

 sphincter nucleus. It has 

 been shown, however, that 

 as the optic tract ap- 

 proaches its termination 

 the visual and the pupil- 

 lary fibers separate and it 

 has been assumed that the 

 latter come into anatomic 

 relation with some inter- 

 calated system which 'in turn is connected with the sphincter nucleus. As 

 to the situation, origin and course of this system nothing positively is known. 

 There is some evidence for the view that this intercalated system has its 

 origin in the anterior corpora quadrigemina as shown in the accompanying 

 diagram. 



Transaction of Spinal Cord 



FIG. 264. DIAGRAM DESIGNED TO SHOW THE MECHAN- 

 ISM OF THE IRIS REFLEX. The central termination of the 

 pupillary fibers is hypothetical. 



