THE GLOBE OF THE EVE. 



933 



which are generally attached to the upper border of the pupil ; on the lower 

 marofin, when present, tliey are much smaller. Their colour is a brownish black. 

 They are sometimes so large as to j^ive rise to apprehensions of injury to the 

 vision.) 



The larger circumference of the iris is attached to the ciliary muscle, which 

 unites it to the choroid ; it is also related to the margin of the cornea, as well as 

 to tliat of the sclerotic opening. 



The lesser, or internal circumference, is elliptical, and circumscribes the 

 pupillary opening. 



Structure. — The organization of the iris has been much discussed ; but at 

 present it is admitted that its principal element is unstriped muscular fibre. A 

 proper membrane and two epithelial layers enter into its formation. 



The proper membrane has, for its framework, circular or radiating fasciculi of 

 wavy connective tissue, with pigment- 

 cells. Between the fasciculi are placed 

 the unstriped fibres ; these are disposed 

 in a circular manner around the pupil 

 to constitute the pup ilia r// sphincter, and 

 others radiate from the lesser circum- 

 ference towards the ciliary ligament to 

 form the dilator of the pupil. Yexy fine 

 radiating vessels are disseminated among 

 these fibres, and pass to the anterior 

 ciliary trunks. The nerves supplied to 

 the iris are from the ophthalmic gang- 

 lion — which has afferent nerves from 

 the ophthalmic nerve of Willis — the 

 common oculo-motor nerve, and the 

 sympathetic. These nerves regulate 

 the reflex movements of contraction 

 a)id dilatation of the pupil. 



(Behind the proper membrane has 

 been described a clear, homogeneous one, 

 composed of dense elastic substance.) 



The anterior epithelial laj/er is composed of the polygonal cells of the aqueous- 

 humour membrane, already described as existing on the posterior surface of the 

 cornea. 



The posterior epithelial layer, or iwea, is constituted by pigment-cells analogous 

 to those of the choroid, but less regular in shape. (This pigmentary stratum — 

 or pars iridica retincB — in addition to the cells, has a fine covering — the limitans 

 iridis — the continuation of the limitans interna retince.) 



In the foetus, the pupil is closed by a very thin transparent membrane — the 

 memhrana pupillaris. (It is identical with the anterior layer of the capsule of 

 the crystalline lens.) 



C. Nerve-Membrane. 

 5. The Retina (Figs. 502, d ; 507 ; 508). 



{Preparation. — The choroi.I must be removed under water by raeans of forceps and ecisaora, 

 after the lens and vitreous humour have been evacuate<l. A ^ood view of the retina is to be had 

 by looking through the vitreous liumour, after the Kns wnd irid have beeu excised from an eye.) 



MUSCrLAR STRUCTURK OF THE IKIS OF K 

 WHITE RABBIT. 



a, Sphincter of the pupil; 6, 6, radiating fasciculi 

 of dilator muscle; c, c, connective tissue, with 

 its corpuscles. 



