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THE STRUCTURE OF THE EYEBALL 511 



the sphincter of the pupil. The dilator muscle of the iris is supplied by 

 nerve fibres which originate in nuclei situated near that part of the 3rd 

 nerve nucleus which supplies the sphincter fibres ; this must be the case 

 in order to explain the reciprocal innervation of the two antagonistic sets 

 of muscles. From these nuclei nerve fibres travel down the cord as far 

 as the 8th cervical and 1st dorsal ventral nerve roots, with which they leave 

 the cord. They then proceed as part of the white rami communicantes to 

 the superior thoracic ganglion, and thence by the sympathetic chain to 

 the superior cervical ganglion, with the cells of which they anastomose. 

 From these nerve cells the terminal nerve fibres for the dilator muscle of the 

 iris arise ; they appear to travel by two distinct routes : (1) from the superior 

 cervical ganglion to the Gasserian ganglion of the 5th nerve, along the nasal 

 branch of its 1st division, to turn off with the two long ciliary nerves to end 

 by entering the sclera and joining the perichoroidal plexus, and thence to 

 the dilator muscle; (2) from the superior cervical ganglion grey rami are given 

 off which travel with and form plexuses on the various branches of the in- 

 ternal carotid artery. One of these is the cavernous plexus which sends 

 a fine branch to the ciliary ganglion. From here the fibres travel with the 

 short ciliary nerves as already described. 



It is clear therefore that the short ciliary nerves contain both pupil- 

 constrictor (3rd nerve) and pupil dilator (sympathetic) fibres, so that when 

 these nerve fibres are stimulated electrically both the sphincter and the radial 

 muscles of the iris will contract. But the sphincter fibres being the more 

 powerful will overcome the others and therefore cause contraction of the 

 pupil. 



The nerve paths above described and the effects on the pupil which 

 excitation of the nerves produces have been ascertained by the employment 

 of the well-known methods of cutting the nerves, stimulating the cut ends, 

 and also by following the tracts marked by degeneration. Thus cutting 

 the sympathetic anywhere causes contraction, while cutting the 3rd nerve 

 produces dilatation. Stimulation of the peripheral cut ends causes the 

 opposite effects. The course of the dilator impulses down the cord to the 

 cervico-dorsal region explains the contraction of the pupil which sometimes 

 accompanies injuries to the cervical spine, and explains the origin of the term 

 cilio-spinal centre. 



Since the dilatation of the pupil is accompanied by contraction of 

 the radial fibres on the one hand, and inhibition of the sphincter fibres on 

 the other, and vice versa when contraction takes place, it would seem 

 almost necessary to assume that there is some system of reciprocal innerva 1 - 

 ion, like that found by Sherrington in the case of the limbs. Experimental 

 evidence would point to such being the case. Thus stimulation of 

 a part of the sensor i- motor area of the brain is followed by dilatation 

 f the pupil, which occurs even when the sympathetic has been 

 cut. Since this excludes the possibility of an active contraction of the radial 

 fibres (their nerve supply having been cut) it appears to prove that a 



iprocal inhibition of the sphincter has been produced. The existence 



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