I45Q 



HUMAN ANATOMY. 



Epithelium 



meridional lamellae. The tissue yields gelatine on boiling. With the fibrous bundles 

 is associated a rich net-work of fine elastic fibers. The clefts between the lamellae 

 contain irregularly stellate connective tissue cells the scleral corpuscle*. On the 

 inner surface of the sclera many of these cells are pigmented and give it a brownish 

 color. This layer the lamina fusca forms with the underlying choroid a narrow 

 lymph-space, the suprachoroidal lymph-space, both walls of which, together with the 

 fine connective tissue trabeculae which cross it, are lined with endothelial cells. The 

 outer surface of the sclera, from the optic nerve entrance to the attachment of the 

 ocular muscles, is similarly covered with endothelial plates, and forms part of the lining 

 of Tenon's lymph-space. Anterior to the muscle-insertions it is covered with a 

 loosely meshed connective tissue, the episcleral tissue, which is richly supplied with 



blood-vessels, nerves and lymph- 



FIG. 1204. vessels, and is continuous with 



the subconjunctival tissue of the 

 conjunctiva sclera. 



The blood-vessels of the sclera 

 arise from the arteries which per- 

 forate it to supply the vascular coat 

 of the eye, viz : the anterior and 

 posterior ciliary arteries. They 

 form a wide meshed net-work on 

 the surface of the sclera, which 

 sends anastomosing vessels to a 

 deeper lying set in the substance of 

 the membrane. In the neighbor- 

 hood of the optic nerve entrance 

 the branches of the short posterior 

 ciliary arteries form an arterial 

 circle, the circulus Zinni, which 

 sends branches to the optic nerve 

 and choroid, and is therefore of 

 great importance in establishing 

 an anastomosis between the cho- 

 roidal circulation and the arteria 

 centralis retinae which supplies the 

 retina. 



The veins of the sclera empty 

 into the anterior and posterior 

 ciliary veins, and into the venae 

 vorticosae. At the junction of the 

 cornea and sclera is an important circular venous channel, the canal of Schleww, 

 which will be described later. The lymphatics of the sclera are represented by 

 the intercommunicating cell-spaces, which communicate with the suprachoroidal 

 and suprascleral lymph-spaces, and anteriorly with the spaces of Fontana, at the 

 sclero-corneal angle. . 



The nerves of the sclera are derived from] the ciliary nerves during their course 



between the sclera and the choroid, their terminal filaments being distributed to the 



vessels, and also as a fine tortuous net-work between the bundles of the scleral tissue. 



The relations of the sclera to the optic nerve sheaths will be considered in the 



description of the optic nerve entrance (page 1470). 



The Cornea. The cornea forms the anterior one-fifth of the fibrous tunic of 

 the eyeball, and, although composed, like the sclera, of bundles of connective tissue, 

 is transparent and allows rays of light to enter the eyeball. Its anterior surface 

 is nearly but not quite circular, measuring 11.9 mm. in its greatest transverse 

 diameter, and n mm. in its vertical diameter. The posterior surface is circular and 

 measures 13 mm. in diameter. The sclera therefore encroaches more upon the cornea 

 anteriorly than posteriorly, so that the cornea fits into a groove in the sclera. The 

 radius of curvature of the anterior corneal surface is about 7.7 mm., that of the hori- 

 zontal meridian being slightly greater (7.8 mm.) than that of the vertical. The 



Posterior 



limiting 

 _metnbrane 

 Endothelium 



Section of human cornea. X 85. 





