1024 



ORGANS OF SPECIAL SENSE 



arteries are too small to be seen in an ordinary examination. The nerves are the 

 long and short ciliary. Nearer the equator, the large venous trunks emerge; they 

 can be traced for some distance in front of their exit as dark lines, running antero- 

 posteriorly beneath the sclerotic. The optic nerve is seen in section, surrounded 

 loosely by a thick outer sheath ; in the centre of the nerve-section a small red spot 

 indicates the position of the central retinal blood-vessels. 



For ordinary dissections eyes of the sheep, pig, or bullock should be obtained. Divide an 

 eyeball into fore and hind halves by cutting through it in the equatorial plane. 



1. Posterior hemisphere seen from in front. This is much the same view that the ophthal- 

 moscope affords us. Unless the eye be very fresh, however, the retina will have lost its trans- 

 parency, and will now present the appearance of a thin whitish membrane, detached in folds 

 from the underlying coats, but still adherent at the optic disc. The vitreous jelly lying within 

 t ho retinal cup may be torn away. In the human eye the retina next the posterior pole is stained 

 yellow (macula lutea). On turning the retina over, a little pigment may be seen adhering 

 to its outer surface here and there. Cut through the retina close to the optic disc all around 

 and remove it: note how easily it is torn. We now see a dark brown surface, consisting of the 

 retinal pigment layer, adherent to the inner surface of the chorioid. Brush off the retinal pig- 

 ment under water. The chorioid thus exposed can for the most part be fairly easily torn away 

 from the thick sclerotic, as a lymph-space exists between them, but the attachment is firm 

 around the optic nerve entrance, and also where the arteries and nerves join the chorioid after 

 penetrating the sclerotic. The chorioid is darkly pigmented, of a brown colour, with markings on 

 its surfaces corresponding to the distribution of its large veins. The inner surface of the sclerotic 

 is of a light brownish colour, mainly from the presence of a delicate pigmented layer, the mem- 



Fio. 725. DIAGRAMMATIC VIEW OF THE INSERTIONS or THE OCULAR MUSCLES. 



(After Merkel.) 



brana suprachorioidea, which adheres partly to it, partly to the chorioid, giving to their adjacent 

 surfaces a flocculent appearance when examined under water. 



2. Anterior hemisphere viewed from behind. The round opening of the pupil is visible in 

 the middle, with the large clear crystalline lens lying nearer us. The retina proper extends 

 forwards a little way from our line of section, and then ends abruptly in a wavy line called the ora 

 serrata, beyond which it is only represented by a very thin membrane (pars ciliaris retinae). 

 Outside the periphery of the lens are a number of ciliary processes arranged closely together 

 in a circle concentric with the pupil, and each radially elongated; posteriorly they are continuous 

 with numerous fine folds, also radial, which soon get very indistinct as they pass backwards, but 

 reach almost to the ora serrata (plicas ciliares). Between the front of the ciliary processes and 

 the edge of the pupil lies the iris. On removal of the retina the inner surface of all this region 

 is seen to be darkly pigmented, but especially dark in front of the position of the ora serrata. 

 Vitreous probably still adheres to the back of the lens, and by pulling upon it the lens can be 

 removed along with its capsule and suspensory ligament; some pigment will now be found 

 adhering to the front of the vitreous, torn from the ciliary processes, which are consequently 

 now lighter in colour than before. The lens-capsule is transparent, and has a smooth glistening 

 outer surface; through it a greyish, star-shaped figure may be observed on the anterior and 

 posterior surfaces oHhe lens. The suspensory ligament is a transparent membrane attached to 

 the capsule of the lens about its equator, and is best seen by floating the lens in water in a glass 

 vessel placed on a dark ground. On opening the capsule we expose the lens itself, which is 

 superficially soft and glutinous to the touch, but becomes firmer as we rub off its outer layers and 

 approach its centre. Carefully tear the chorioid and iris from the sclerotic as far as possible: 

 a firm adhesion exists just behind the corneal periphery. The outer surface of the chorioid 

 thus exposed is found to be also rather darkly pigmented, hut it shows a white ring correspond- 

 ing to the adhesion just mentioned, and a pale area behind this ring indicates the position of 



