1022 ORGANS OF SPECIAL SENSE 



depressed meshes at intervals. The radial ridges coming from the edge of the 

 pupil, and those coming from the more peripheral part of the iris, meet in a zigzag 

 elevated ridge concentric with the pupil, called the corona iridis, and by this ridge 

 the iris is roughly marked off into two unequal zones an outer, the greater or ciliary, 

 and an inner, the lesser or pupillary of which the inner is much the narrower. 

 The border next the pupil is edged with small, roundish, bead-like prominences of 

 a dark brown colour, separated from one another by depressions, so that it presents 

 a finely notched contour. Not infrequently, in a light-coloured iris, we may see the 

 sphincter muscle through the anterior layers, in the form of a ring about one milli- 

 metre hi breadth around the pupil. The ciliary zone may be described as con- 

 sisting of three parts: (a) A comparatively smooth zone next the zigzag ridge; 

 (b) a middle area, showing concentric but incompletely circular furrows; (c) a 

 small peripheral darker part, presenting a sieve-like appearance. On the floor of 

 the large depressed meshes, or crypts, parallel radial vessels can be traced, belonging 

 to the iris-stroma. The zigzag line mentioned above corresponds to the position 

 of the circulus arteriosus minor. Occasionally, especially in a light iris, super- 

 ficial pigment spots of a rusty brown colour occur. 



If we are examining the living eye, the ophthalmoscope should now be used, so as to gain a 

 view of the fundus. We can thus study the termination of the optic nerve, the distribution of 

 the larger retinal vessels, etc. 



The general red reflex obtained from the fundus is due to the blood in a capil- 

 lary network (chorio-capillaris) situated in the inner part of the chorioid. To 



FIG. 724. LEFT FUNDUS OCULI , AS SEEN BY DIRECT OPHTHALMOSCOPIC METHOD. 



LAMINA CRIBROSA 



MACULA LUTEA AND FOVEA CENTRALIS 



PIGMENT AT OUTER BORDER OF DISC 



the nasal side of the centre of the fundus is a paler area of a disc shape corresponding 

 to the intraocular end of the optic nerve, and known as the papilla of the optic 

 nerve or optic disc. This optic disc is nearly circular, but usually slightly oval 

 vertically; it is of a light orange-pink colour, with a characteristic superficial trans- 

 lucency; its outer third segment is paler than the rest from the nerve-fibres and 

 capillaries here being fewer. About its centre we often observe a well-marked 

 whitish depression or gap, formed by the dispersion of the nerve-fibres as they spread 

 out over the fundus; at the bottom of this depression a sieve-like appearance may 

 be seen, due to the presence of the lamina cribrosa, which consists of a white fibrous 

 tissue framework, with small, roundish, light-grey meshes in it, through which 

 latter the nerve-fibre bundles pass. Also near the centre of the disc, the retinal 

 blood-vessels first come into view, the arteries narrower in size and lighter in colour 

 than the veins; they divide dichotomously as they are distributed over the fundus. 

 The retina proper is so transparent as to be ophthalmoscopically invisible, but its 

 pigment-epithelium gives a very finely granular or darkly stippled appearance to 

 the general red reflex. In the centre of the fundus. and therefore to the outer side 

 of the disc, the ophthalmoscope often shows a shifting halo of light playing round 

 a horizontally oval, comparatively dark enclosed area; this latter corresponds to 

 the yellow spot region, and about its centre a small pale spot usually marks the 

 position of the fovea centralis. 



Two structures visible at the nasal end of the palpebral aperture have been 

 previously mentioned, and should now be examined more narrowly. The lachry- 



