892 



THE ORGANS OF SENSE. 



the centre of the lamella ; it is called the porus opticus, and transmits the arteria 

 centralis retinae' to the interior of the eyeball. Around the cribriform lamella are 

 numerous small apertures for the transmission of the ciliary vessels and nerves 

 In front the sclerotic is continuous with the cornea by direct continuity of tissue, 

 but the opaque sclerotic overlaps the cornea rather more on its outer than on ita 

 inner surface. 



Structure. The sclerotic is formed of white fibrous tissue intermixed with fine 

 elastic fibres, and of flattened connective- tissue corpuscles, some of which are 

 pigmented, contained in cell-spaces between the fibres. These fibres are aggre- 

 gated into bundles which are arranged chiefly in a longitudinal direction. It yields 

 gelatin on boiling. Its vessels are not numerous, the capillaries being of small 

 size, uniting at long and wide intervals. The existence of nerves in it is doubtful. 



Canal of Schlemm. 



Retina. 



Choroid coat. 



Sclerotic coat. 



Nerve sheath 



Canal for 

 entral artery. 



! ' vrJ Optic nerve. 



FIG. 527. A horizontal section of the eyeball. (Allen.) 



The Cornea is the projecting transparent part of the external tunic of the 

 eyeball, and forms the anterior sixth of the globe. It is almost circular in shape, 

 occasionally a little broader in the transverse than in the vertical direction. It 

 is convex anteriorly, and projects forward from the sclerotic in the same manner 

 that a Avatch-glass does from its case. Its degree of curvature varies in different 

 individuals, and in the same individual at different periods of life, it being more 

 prominent in youth than in advanced life, when it becomes flattened. The 

 cornea is dense and of uniform thickness throughout ; its posterior surface is 

 perfectly circular in outline, and exceeds the anterior surface slightly in extent, 

 from the latter being overlapped by the sclerotic. 



Structure. The cornea consists of four layers namely, (1) several strata 

 of epithelial cells, continuous with those of the conjunctiva ; (2) a thick central 

 fibrous structure, the cornea proper; (3) a homogeneous elastic lamina; and (4) 

 a single layer of epithelial cells, forming part of the lining membrane of the 

 anterior chamber of the eyeball. The name of membrane of Descemet or Demours 

 is given to this posterior elastic lamina and its endothelial coating. 



