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TEXT-BOOK OF PHYSIOLOGY 



diminishes in size, in consequence of the contraction of the sphincter pupilla 

 caused by a stimulation of the peripheral ends of the pupillary fibers of the 

 retina, the degree of contraction depending within limits on the intensity 

 of the light. 



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; an emissive center, 

 the sphincter nucleus of 

 the motor oculi center; effer- 

 ent nerves, including fibers 

 in the trunk of the motor 

 oculi and in the ciliary 

 nerves; and a responsive 

 organ, the muscle. (See 

 Fig. 268). That this is the 

 mechanism involved in this 

 reflex, is shown by the fact 

 that when any portion 

 of it is destroyed, the 

 reflex contractions 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 

 FIG. 268. DIAGRAM DESIGNED TO SHOW THE MEGHAN- has been assumed that the 

 ISM OF THE IRIS REFLEX The central termination of the j ^ ^to anato mic 



pupillary fibers is hypothetical. . J 



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. 



The contraction of the sphincter and a diminution in the size of the pupil 

 may be direct, as when the light which enters one eye causes a reflex contrac- 

 tion of the sphincter of one and the same side; or it may be indirect or 

 consensual, as when the light, which enters one eye only, causes a contraction 



Sup. Cervical Ganglion, 

 fibers^ 



JF^Thoracic Newt 



Transection of Spinal Cord 



