THE EYEBALL 



1055 



meshes, or crypts, parallel radial vessels can be traced, belonging to the iris-stroma. The zig- 

 zag line mentioned above corresponds to the position of the circulus arteriosus minor. Occa- 

 sionally, especiall}' in a light iris, superficial pigment spots of a rusty brown colour occur. 



(In examining the living eye, the ophthalmoscope may now be used, so as to gain a view 

 of the fundus, and to 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 capillary network 

 (chorio-capillaris) situated in the inner part of the chorioid. To 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 [papilla n. optici]. This papilla (or 'optic disc') 

 is nearly circular, but usually slightly oval vertically; it is of a light orange-pink colour, with a 

 characteristic superficial translucency; its lateral third segment is paler than the rest as nerve- 

 fibres and capillaries here are fewer in number. About its centre we often observe a well- 

 marked whitish depression [excavatio papilla' n. optici], 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 sclerse, which consists of a white fibrous 

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

 bundles pass. Also near the centre of the papilla, 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 ophthal- 

 moscopically 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 lateral 

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



Fig. 799. — The Normal Fundus of the Eyeball. (Parsons 



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

 spot [macula lutea] 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 lacrimal caruncle is an island 

 of modified skin, and fine hairs can commonly be detected on its surface, and it contains sebaceous 

 and sweat glands. Lateral to it and separated from it by a narrow groove, is the semilunar 

 fold of conjunctiva; it rests on the eyeball, and is a rudiment of the third eyelid or nictitating 

 membrane, present in birds and well represented in many other vertebrates. 



Examination of the Eyeball 



(In the following account, the structure of the eyeball is described as it would 

 appear upon dissection.) 



The eyeball [bulbus oculi] is almost spherical, but not perfectly so, mainly be- 

 cause its anterior, clear, or corneal segment has a greater curvature than the rest of 

 the eye. Considering it as a globe, it has an anterior pole [polus anterior] and 

 a posterior pole [polus posterior]; the former corresponding to the centre of the 

 front of the cornea, the latter to the center of the posterior curvature. An imag- 

 inary straight line joining the two poles is called the axis of the eyeball. The 

 equator of the eye is that part of its surface which lies midway between the two 

 poles. The various meridians are circles which intersect the poles. The sagittal 

 axis of the globe is the greatest (about 24. 5 mm.), the vertical equatorial the least 



