534 



TEXT-BOOK OF PHYSIOLOGY. 



The majority of the ocular movements, the power of accommoda- 

 tion, the variations in the size of the pupil in accordance with varia- 

 tions in the intensity of the light, the power of convergence of the 

 visual axes, are all excited by the transmission of nerve impulses by 

 the constituent fibers of the nerve from their related nuclei. This 

 is made evident by the effects which follow stimulation and division 

 of the nerve or lesions of the nuclei themselves. 



The central nuclei can be excited to activity (i) by nerve impulses 

 descending the motor tract, from the cerebral cortex, (2) by nerve 

 impulses coming through various afferent nerves. This holds true 

 more especially for the sphincter pupillae nucleus. 



The Iris Reflex or the Pupillary Reflex. These are terms 

 applied to the variations in the size of the pupil that follow vari- 



GasserianGanglion 



Sympathetic. 

 anglionic 

 fibers 



Superior /'j > 

 Cervical [ / \ 

 Ganglion 



1 ThoracicNeroe\ 



FIG. 245. DIAGRAM SHOWING THE STRUCTURES INVOLVED IN THE IRIS REFLEX. 



ations in the intensity of the light. In the absence of light the 

 pupil widely dilates, due largely to the relaxation of the sphincter 

 pupilla muscle and partly to a contraction of the radiating fibers of 

 the iris which collectively constitute the dilatator pupilla muscle. 

 With the entrance of light into the eye, the pupil narrows in con- 

 sequence of the contraction of the sphincter pupillce caused by a 

 stimulation of the peripheral end of the optic nerve, the degree 

 of contraction depending within limits on the intensity of the light, 

 ^^^he action is a reflex one and the mechanism involved includes 

 the retina, the optic nerve, the anterior quadrigfeminal body, the third 

 nerve, the ciliary nerves (the peripheral sympathetic neurons), and 

 the sphincter pupillae muscle. ( Fig. 245^^111 this mechanism the 

 optic nerve is the afferent path, the motor oculi the efferent path, 

 and the anterior quadrigeminal body the intermediate center^These 



