THE CRANIAL NERVES. 535 



facts are demonstrated by the entire loss of the reflex which fol- 

 lows division or destruction of any part of this arc. The anterior 

 quadrigeminal body appears to be in relation through its axons and 

 their collateral branches with the sphincter pupillce nucleus of both 

 sides, inasmuch as stimulation of one retina is followed by narrow- 

 ing of the pupils of both eyes. To this simultaneous contraction of 

 the pupils the term " consensual" has been given. Contraction of 

 both pupils also occurs as an associated movement in the conver- 

 gence 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 dila- 

 tator pupillse muscle, which is kept normally in a state of tonic 

 contraction 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. 245), 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 

 sensory nerves. That the afferent 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 dilata- 

 tion of the pupil. 



The variations in tfrf gj* gf tf>p pupil, though largely a reflex 

 act unjej the rn-ntrnl nf the..._n_ni1nrmQtnr-jaegv^ 7 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 Pupillary Reaction. It was stated on page 531 

 that a modification of the pupillary reaction is observed in some 

 cases of hemianopsia which indicates approximately the seat of lesion. 

 This reaction is present only when the lesion is along the course of 

 the optic tract. In these cases, if a fine ray of light is projected into 

 the eye in such a manner that it falls entirely on the non-responsive 

 side of the retina, there will be an absence of a pupillary response, 

 owing to the break in the reflex arc. If, however, the light be 



