THE CRANIAL NERVES. 583 



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 

 of the sphincter not only in the eye of the same, but in the eye of the opposite 

 side also. It is, however, highly probable that all reflex contractions of 

 the sphincter mu'scles are consensual, that is, bilateral reflex actions because 

 of the decussation of the pupillary fibers at the chiasm. Contraction of both 

 pupils also occurs as an associated movement in the convergence of the eyes 

 during accommodation. 



The dilatation of the pupil is, however, not due exclusively to the 

 relaxation of the sphincter pupillae muscle, but partly to the contraction of 

 the dilatator pupillae muscle, which is kept normally in a state of tonic con- 

 traction by impulses emanating from a nerve-center in the medulla 

 oblongata. 



The axons which arise in this center pass down the cord, emerge through 

 the first thoracic nerve, and then ascend to the superior cervical ganglion 

 (see Fig. 270), in which their terminal branches arborize around its nerve- 

 cells. From these cells new axons of the sympathetic system arise which pass 

 successively to the ophthalmic division of the fifth nerve, the nasal nerve, 

 the long ciliary nerve and the iris. 



Experimental research renders it highly probable that the dilatator 

 center is in a state of continuous activity and the dilatator muscle in a state 

 of tonic contraction. Whatever the normal stimulus may be, the center 

 is increased in activity by dyspneic blood, by severe muscle exercise, by 

 emotional excitement, and by stimulation of various sensor nerves. That 

 the efferent pathway just alluded to transmits the impulses to the iris is shown 

 by the fact that division in any part of the course is followed by narrowing, 

 stimulation by active dilatation of the pupil. 



The variations in size of the pupil, though largely a reflex act under the 

 control of the oculo-motor nerve, are nevertheless partly due to the active 

 cooperation of the dilatator nerves and their related muscle. The size of the 

 pupil necessary from moment to moment for the admission of just that 

 amount of light essential to the formation and perception of a distinct image 

 is the result of two nicely adjusted and delicately balanced forces. 



Wernicke's Hemianopic Pupillary Reaction. It was stated on 

 page 578 that a modification of the pupillary reaction is observed in some 

 cases of hemianopsia, which indicates approximately the seat of the lesion. 

 This reaction, or inaction as it is sometimes called, is present when the lesion 

 is along the course of the optic tract between the chiasma and the anterior 

 quadrigeminal body. In a case of left lateral hemianopsia, the lesion being 

 in the right optic tract, the method of testing for the reaction is as follows: 

 The eye of the left side is first carefully shielded from th e light. A fine ray 

 of light is then projected into the right eye in such a manner that it falls en- 

 tirely on the non-sensitive (the temporal) side of the retina. There will be an 

 absence of the usual pupillary response, or rath er the pupil remains inactive; 

 but if the light is gradually directed toward the sensitive (the nasal) side of 

 the retina, there will come a moment, as t he central line is crossed and the 

 light falls on the sensitive side, whe n the usual pupillary response manifests 



