•lOG THE SKIN AND THE SENSE-OliGANS. 



fourths of the surface of the eyeball ; posteriorly it is pierced by the 

 optic nerve at a point [poms opticus) nearer the temporal side than the 

 nasal. The sclerotic coat consists of fibrous tissue externally, with a 

 layer of hyaline cartilage internally (Helfreich). The fibrous layer 

 Pj 2C3. is formed of bundles of j)arallel fibres, which cross 



each other, chiefly at rig-ht angles (Hoffmann). 

 The cartilaginous layer ends just behind the line 

 of insertion of the extrinsic muscle" of the eye, 

 and is thickest at the point of entrance of the 

 optic nerve (Helfreich). 



The sclerotic coat is rich in nerve-fibres, which 



Endothelium from the p i iijici i i 



inner surface of the lomi a closc nctwork ; tlic u Dres, liowcver, do 

 sclerotic coat ; after ^q^ uuitc but f orm the meslics of the network 



Hoflfmann. 



by simply crossing each at acute angles. 

 The deeper surface of the sclerotic coat is lined with a layer of 

 larg-e endothelial cells (Hoffmann), (Fig. 253), which form the 

 outer wall of the capsule of Tenon. 



b. The cornea and the anterior chamber. The cornea forms 

 about one-fourth of the surface of the eyeball and is directly con- 

 tinuous with the sclerotic. In it five layers can be distinguished : 

 a layer of stratified epithelium or conjunctiva, an anterior hyaline 

 membrane, the true corneal substance, a posterior hyaline membrane, 

 and a layer of endothelium. 



(i) The corneal epithelium is a layer of stratified e^^ithelium 

 covering the superficial surface of the cornea. The superficial 

 layer forms a beautifid mosaic of polygonal cells; the middle 

 layers are polygonal in all sections, while the deepest layer is more 

 or less columnar. Excei:)t in the most superficial layer, all the cells 

 have serrated surfaces. Smaller cells possessing each two nuclei 

 are also found between the columnar cells, and are evidently cells in 

 process of division ; according to Waldeyer, cell-proliferation may 

 also take place in the middle layers. 



The basal or deeper portions of the columnar cells possess a clear 

 border, which reminds one of the hyaline border found on the free 

 border of columnar epithelium in other parts. The cells are here so 

 closely applied to one another that these borders have the appearance 

 of a continuous, highly refracting membrane (Rollett) ; according 

 to Henle, the border consists of a network of very fine processes from 

 the cells above. 



(2) The true corneal substance, and (3) the anterior hya- 



