SIGHT. 



739 



layer which is the proper tissue of the organ. This middle layer is com- 

 posed of about sixty superimposed laminae of fusiform fibrous cells. In the 

 interstices between the laminae are found tubular spaces, which contain a 

 transparent fluid. The anterior and posterior elastic laminae are structure- 

 less and highly elastic. When separated from the proper corneal tissue they 

 have a great tendency to curl up, suggesting that they are active agents in 

 the retention of a proper curvature of the cornea. The cornea is covered 

 on its anterior surface by the conjunctival mucous membrane, which consists 

 of three or four layers of pavement epithelial cells ; the deeper layers of 

 cells are oblong, and placed perpendicularly. The conjunctiva at this point 

 has no perceptible basement membrane. The posterior surface of the cornea 

 is covered by a transparent serous membrane, which consists of a simple 

 layer of polygonal pavement epithelial cells resting on an elastic mem- 



FIG. 157. 



Diagram of a Horizontal Section of the Eyeball a, outer or sclerotic coat ; d, the cornea ; 6, 

 middle or choroidal coat ; m, ciliary ligament ; s, ciliary process ; e, ciliary muscle, and/, iris ; c, 

 inner coat of retina, continuous with the optic nerve behind, with a dark layer outside it ; g, lens ; 

 t, suspensory ligament of the lens ; h, vitreous body n, hyaloid membrane ; i, posterior chamber ; 

 o, canal of Petit ; r, sinus circularis iridis ; I, optic nerve. The dotted line through the centre is 

 the longitudinal axis of the ball. 



cornea 



has 



no 



brane. This is called the membrane of Demours. The 

 bloodvessels, and therefore derives its nutriment by diffusion. 



The sclerotic coat is so named on account of the firmness of its texture and 

 of its hardness. It forms the outer tunic of the posterior segment. It is 

 whitish, opaque, smooth, excepting at the points of attachment of the muscles 

 of the eyeball. It is composed of white fibrous tissue, arranged more or less 

 in bundles, which interlace each other in various directions. Anteriorly the 

 interlacements are in a generally transverse direction ; posteriorly the direc- 

 tion is longitudinal. This coat also contains yellow elastic fibres and 

 fusiform nucleated cells. It is continuous anteriorly with the cornea, and 

 posteriorly with the perineurium of the optic nerve. At the internal border 

 of the junction with the cornea is a venous sinus called the sinus circularis 

 iridis or canal of Schlemm. The optic nerve pierces it about 2.6 mm. internal 

 to the antero-posterior axis of the eyeball. At this point the coat is perfo- 



