320 PHYSIOLOGY CHAP. 



that the former contracts the sphincter and relaxes the dilatator, 

 while the latter contracts the dilatator and relaxes the sphincter. 



In explaining myosis and mydriasis some authors particularly 

 Griinhagen have ascribed great importance to the vaso-inotor 

 nerves to the iris that accompany the ciliary nerves. They hold 

 that independently of the variations in the tone of the muscles of 

 the iris, vascular constriction and anaemia are capable of producing 

 mydriasis, while vascular dilatation and hyperaemia may produce 

 myosis. But this theory is contrary to certain definite experi- 

 mental facts : (a) mydriasis may be produced by excitation of the 

 sympathetic even when the vessels have been emptied by profuse 

 bleeding, or the arteries of the neck tied ; (&) the constriction of 

 the iridic vessels obtained by excitation of the carotid fibres of 

 the sympathetic above the upper cervical ganglion does not alter 

 the diameter of the pupil (Frangois-Franck) ; (c) the mydriasis 

 consequent on excitation of the sympathetic is not synchronous 

 with, but precedes, the constriction of the vessels of the iris (Arlt 

 and others) ; (d) stimulation of the carotid fibres of the rabbit's 

 sympathetic, which produces an obvious ischaemia of the iridic 

 vessels, not merely fails to produce mydriasis, but even causes a 

 slight degree of myosis (Angelucci). 



The determination of the centripetal paths and centres for the 

 constrictor and dilatator reflexes of the pupil is based particularly 

 on clinical observations associated with anatomo - pathological 

 discoveries. 



The centripetal paths for the pupil reflexes that occur on 

 stimulating with light are represented by the fibres of the optic 

 nerve, which undergo partial decussation in the chiasma (Vol. III. 

 p. 492). It is a remarkable fact that even when the chiasma has 

 been divided in the median line in monkeys, not only the direct 

 but also the consensual pupil reflexes are preserved (Bernheimer, 

 1898). 



Clinical .observations on disease of the terminal nuclei of the 

 fibres of the optic nerve tend to show that the pupil reflex is 

 served by optic fibres other than those which subserve vision. 



In the mesencephalon, and particularly in the anterior corpora 

 quadrigemina, the centripetal paths of the pupil reflex come into 

 relation with the cells of the oculo-motor centre, and from there 

 the centrifugal paths proceed to the sphincter of the pupil and 

 ciliary muscle. According to Bernheimer, on the contrary, the 

 optic fibres concerned in this reflex are directly connected with 

 the oculo-motor nucleus, but the anatomical observations on 

 which he bases his theory have not been confirmed by other 

 authors. On the other hand, Bach and Majano, on the strength 

 of other anatomical observations, hold that the optic fibres 

 terminate in the anterior quadrigeminal bodies, and from these 

 fibres pass directly into the oculo-motor nerve, unconnected with 



