THE SENSES 



909 



received the name of the cilio-spinal region from its relation to the 

 pupillo-dilator fibres. It must not be looked upon as a centre in any 

 proper sense of the term, but rather as the pathway by which these 

 fibres pass down from the bulb, and where they may accordingly be 

 tapped by stimulation. 



Stimulation of certain areas on the cortex of the frontal lobe of the 

 cerebrum (p. 889) causes slight dilatation of the pupil even after the 

 sympathetic has been divided. This is due to inhibition of the pupillo- 

 constrictor fibres in the third nerve. 



Changes in the Pupil during Accommodation. It has been 

 already mentioned that along with the alteration in the curvature 

 of the lens a change in the diameter of the pupil takes place in 

 accommodation. When a distant object is looked at, the pupil 



h 



FIG. 388. 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 pregangli- 

 onic 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 gan- 

 glion into the ophthalmic division (Oph.) of the 

 fifth nerve, V, and 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 pupillae, 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. 



becomes larger ; when a near object is looked at, it becomes smaller. 

 Narrowing 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. Stimu- 

 lation 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. 388), 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 



