THE CORNEA, IRIS AND AQUEOUS HUMOR 817 



wave-length, a greater effort at accommodation is required in order 

 to bring them to a precise focal point upon our retina. 



Miosis and Mydriasis. — These terms are commonly employed to 

 indicate that the size of the pupil has been varied by means of drugs or 

 in consequence of pathological lesions. Miosis signifies pupillar con- 

 striction, and mydriasis, pupillar dilatation. The first condition is 

 commonly associated with congestion and traumas of the iris, certain 

 fevers, pulmonary congestion, and lesions of the sympathetic system. 

 Among the miotics might be mentioned physostigmin (eserin), 

 muscarin, and pilocarpin. Their action appears to be due to their 

 power of stimulating the nerve fibers and corresponding receptor 

 substance of the constrictor muscle. The mydriatics commonly made 

 use of, are atropin, homatropin and cocain. The first two act by 

 paralyzing the endings or receptor substance of the constrictor nerve 

 fibers.^ Cocain exerts a similar action, but only in larger doses, while 

 in smaller doses, it stimulates the dilator mechanism. Mydriasis is 

 also obtained in glaucoma, atrophy of the optic nerve and orbital 

 diseases. , 



A mydriatic eye must, of course, be shielded against light, because 

 it is temporarily unable to protect itself. In addition, it should be 

 remembered that the mydriatics temporarily destroy the mechanism 

 of accommodation, because they paralyze the ciliary muscle which is 

 similarly innervated. Near vision, therefore, is practically impossible 

 at this time. The miotics, on the other hand, also stimulate the ciliary 

 muscle and kedp the eye in a condition of forced accommodation. In 

 the latter case, therefore, far vision is practically impossible. 



The Innervation of the Iris. — The circular and radial fibers of the 

 iris receive their nerve supply from the autonomic system, the relay 

 stations nearest them being the ciliary ganglion and the superior cervi- 

 cal ganglion. Preganglionically, however, these fibers find their origin 

 in the cerebrospinal system. As far as the sphincter iridis is con- 

 cerned, it may be shown that its nerve fibers arise in the midbrain in 

 the anterior part of the nucleus of the third cranial nerve. They make 

 use of the third nerve as a highway to reach the ciliary ganglion, whence 

 Ihey continue onward postganglionically in the short ciliary nerves. 

 The nerve fibers innervating the dilator mechanism of the iris, sup- 

 posedly the radial muscle fibers, also arise in the midbrain, but their 

 place of origin is not definitely known. From here they descend in 

 the spinal cord, but leave this structure in the eighth cervical and the 

 first and second thoracic spinal nerves to enter the sympathetic system 

 by way of the rami albi communicantes. They then ascend to the 

 superior cervical ganglion by way of the cervical sympathetic nerve 

 and finally reach the Gasserian ganglion. Distally to this point they 

 invade the ophthalmic branch of the fifth cranial nerve and its long 

 c-iliary branch. y 



1 Langley, Jour, of Physiol., xxxix, 1909, 235. 



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