THE SENSES 



ing of the pupil is thus associated with contraction of the ciliary 

 muscle, and widening of the pupil with its relaxation. 



This physiological correlation has its anatomical counterpart ; for the 

 third nerve supplies both the iris and the ciliary muscle. Stimulation 

 of the nerve within the cranium causes contraction of the pupil, while 

 stimulation of certain portions of its nucleus in the floor of the third 

 ventricle and the Sylvian aqueduct or of the short ciliary nerves 

 (Fig. 422), which receive branches from the third nerve, or of the 

 ganglion itself, is followed by that change in the anterior surface of the 

 lens which constitutes accommodation (Hensen and Voelckers). This 

 can be observed either through a window in the sclerotic in a dog or by 

 following the movements of a needle thrust into the eyeball. By 

 carefully localized stimulation near the junction of the aqueduct with 



in 



Fig. 422. Scheme of Innervation of Ciliary 

 and Iris Muscles (after Schultz). i, ciliary 

 ganglion; 2, oculo-motor nucleus; 3, spinal cell, 

 from which comes off the preganglionic fibre on 

 the pupillo-dilator path, which forms a synapse 

 with 4, a cell in the superior cervical ganglion. 

 The axon of 4 is shown passing (as an interrupted 

 line) through the Gasserian ganglion into the 

 ophthalmic division (Oph.) of the fifth nerve, V, 

 iid thence in a long ciliary nerve, 5, to the dilator of the iris, 8. From i axons 

 are shown passing by short ciliary nerves to the ciliary muscle, 6, and the constrictor 

 pupilleB, 7; 9, cell of origin (in mid-brain ?) of fibre which constitutes the central 

 neuron of the pupillo-dilator path; 10, optic nerve: III, third nerve; V, fifth nerve 

 with Gasserian ganglion. 



the third ventricle, it is possible to bring about the forward bulging of 

 the lens without any change in the iris ; but the normal and voluntary 

 act of accommodation cannot be disjoined from the corresponding 

 alterations in the size of the pupil. Inward rotation of the eyes accom- 

 panies contraction of the pupil hi accommodation, and the question 

 may be raised whether the pupillary change is associated with the action 

 of the extrinsic muscles of the eyeball which cause convergence or with 

 the action of the intrinsic muscles which determine the changes in the 

 curvature of the lens. It is usually considered to be associated with 

 both. In any case, actual convergence is not necessary for the reaction, 

 since it may still be obtained on accommodation when convergence is 

 impossible on account of paralysis of the internal recti. 



