EXAMINATION OF THE EYEBALL 843 



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

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

 hlood-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 tinely granular or darkly stippled appearance to 

 the general red retiex. 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 

 l)reviously mentioned, and should now be examined more narrowly. The 

 lachrymal caruncle is in reality an island of modified skin, and fine hairs can 

 connnonly be detected on its surface. On its outer side, 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 



The eyeball of a cadaver should now be removed by snipping with scissors the conjunctiva 

 near the corneal border, then cutting through the ocular muscles near their insertion into the 

 gl(jbe, and finally dividing the optic nerve close to the sclerotic. 



The eyeball is almost spherical, but not perfectly so, mainly because its an- 

 terior, clear, or corneal segment has a greater curvature than the rest of the eye. 

 Considering it as a globe, however, we speak of an anterior and of a posterior 

 pole ; the former corresponding to the middle of the front of the cornea, the 

 latter to the middle of the posterior curvature. An imaginary straight line joining 

 tin- two poles is called the antero-posterior or sagittal axis of the eyeball. The 

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

 poles. The sagittal axis of the globe is the greatest (about 24 "o mm.), the ver- 

 tical equatorial the least (about 23*5 mm.), and the transverse equatorial axis is 

 intermediate in length (about 23-9), so that the eyeball is in reality an ellipsoid, 

 flattened slightly from above dow^nwards. Again, if the globe is divided in its 

 antero-jxjsterior vertical plane, the nasal division will be found to be slightly 

 smaller than the temporal. The optic nerve joins the globe three or four milli- 

 metres to the nasal side of the posterior pole. 



The shape of the eye depends on, and is preserved by, the outermost tunic, 

 formed conjointly by the cornea and sclerotic, the entire outer surfaces of which 

 are now in view. The anterior or corneal part has been already examined. All 

 round the cornea there remains a little adherent conjunctiva; elsewhere, the 

 sclerotic is directly exposed, except for some loose connective tissue which adheres 

 to it, especially around the optic nerve entrance. In front of tlie equator we see 

 the tendinous insertions of the four recti muscles. Behind the equator are the 

 insertions of the two oblique muscles — that of the superior ol)lique tendinous, 

 and further forwards; that of the inferior more fleshy, and placed between the optic 

 nerve and the external rectus. 



It is difficult to recognise the different recti muscles by their insertions if we do not know 

 whether the eye examined is a right or a left one. To determine tliis we should liold the globe 

 with the optic nerve towards us, and in the natural position with the su|)erior ob]i<|ue tendon 

 uppermost. The inferior oblique tendon will now point to the side to which the eye belongs, and 

 we can consequently determine the different recti muscles. 



The internal rectus is inserted nearest (7 nun. from) the corneal border; the 

 external rectus commonly, sometimes the sujierior, is inserted furthest from it 

 (about 8 nun.). All the recti tendons are broad and thin, but that of the internal 

 is the broadest (8 mm.); those of the external and inferior the narrowest (6 mm.) 



