DIOPTKIC MECHANISM OF THE EYE 



319 



type the latent period is rather shorter, the dilatation of the pupil 

 more rapid, and the return to its initial size is rapid at first, and 

 then very slow and gradual. In the second type the latent period 

 is longer ; the initial dilatation is followed by sudden constriction 

 at the close of the stimulation ; then follows a secondary dilatation, 

 which is more ample and prolonged ; finally there is a slow and 

 gradual constriction until the normal diameter is regained. This 



FIG. 144. Diagram of direct pupillary dilatation. (After Bellarminoif.) a, pupil after section of 

 the cervical sympathetic ; b, maximal dilatation on stimulation of the sympathetic ; c, pro- 

 gressive contraction till pupil recovers its original diameter ; m, n, duration of electrical 

 stimulation ; p, q, latent period ; s, s, seconds. 



varying course of the pupil reaction is the external expression of 

 the active intervention of the reflex centres. 



How does the oculo-motor nerve produce myosis, and the sympa- 

 thetic mydriasis ? A definite answer to this question is not easy. 

 If we consider the sphincter alone as the prevailing muscle of the 



Fir;. 145. Diagram of reflex pupillary dilatation. (Bellarminoff.) a, pupil after section of sciatic 

 nerve ; b, primary dilatation during stimulation of central end of the sciatic ; c, contraction of 

 the pupil after stimulation ; d, secondary dilatation ; e, moment of maximal dilatation ; 

 m, n, duration of electrical stimulation ; p, q, latent period ; s, s, seconds. 



iris it may be concluded that the oculo-motor by means of the 

 cells of the ciliary ganglion and the short ciliary nerves causes 

 the sphincter to contract, while the sympathetic by means of the 

 superior cervical ganglion and the long ciliary nerves inhibits its 

 tone, or causes active expansion. But in view of the anatomical 

 and physiological evidence that the iris is also provided with a 

 dilatator muscle, the action of which is antagonistic to that of the 

 sphincter, one is forced to conclude that the oculo-motor and the 

 sympathetic fibres have an opposite action upon the two muscles 



