842 ORGAyS OF SPECIAL SENSE 



a clear fluid, the aqueous humour. Almost the whole anterior surface of the iris 

 is visible, its extreme periphery only being concealed by sclerotic. In colour the 

 iris varies greatly in different individuals. Near its centre (really a little up and 

 in) a round hole exists in the iris, the black ])ni)il, whose size varies consideraljl}^ 

 in different eyes, and in the same eye according to temporary conditions, such as 

 exposure to light, etc. 



Ill exaiuiiiing the surface-markings of the living iris, one of dark colour is to be preferred. 

 Focal illumination will be found useful, for which purpose a second convex lens will be required. 



On the surface of the iris we see a number of ridges running more or less 

 radially; adjoining ones occasionally unite and interlace to some extent, so as to 

 leave large depressed meshes at intervals. The 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, and by this ridge the iris is roughly 

 marked off into tAVO unequal zones — an outer or ciliary, and an inner 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 Si3hincter muscle through the 



Fig. 473. — Left Fundus Oculi, as seex by direct Ophthalmoscopic Method. 



Retinal artery 

 Retinal vein 



LAMINA CRIBROSA 

 FOVEA CENTRALIS 

 PIGMENT AT OUTER BORDER OF DISC 



anterior layers, in the form of a ring about one millimetre in breadth around the 

 pupil. The ciliary zone may be described as consisting of three parts: (a) A 

 comparatively smooth zone next the zigzag ridge; (h) a middle area, showing 

 concentric but incompletely circular furroAvs; (c) a small peripheral darker part, 

 presenting a sieve-like appearance. On the tloor 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, superficial 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 ftmdus. 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 clioroid. To 

 the nasal side of the centre of tlie fundus is a paler area of a disc shai)e corre- 

 S])()nding to the intraocular end of the optic nerve, and known as the optic disc, 

 or papilla. This optic disc is nearly circular, but usually slightly oval vertically; 

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

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

 here being fewer. About its centre we often observe a Avell-marked whitish depres- 

 sion 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 



