THE FIBROUS TUNIC 9 



SO. Despite this easily ascertained fact, many speculations have been 

 made as to what factor is responsible for the transparency of the cornea 

 and the lens. The really interesting question is, what makes the other 

 tissues of the developing embryo become opaque. 



The sclera (Fig. 4a, s) is composed of tough, inelastic, tendinous tissue 

 organized in ribbon-like bundles of microscopic fibers which are felted 

 together in such a way that the whole tissue is about equally strong in all 

 directions — to resist the intraocular pressure, equal of course in all direc- 

 tions, without allowing the eyeball to change its shape. The flat fiber- 

 bundles are of unknown length, for their ends cannot be found; but each 

 seems to arise somewhere behind the rim of the cornea, runs parallel 

 thereto for a space, then courses backward around the eye and forward 

 again in a wide loop — not, however, following a great circle of the ocular 

 sphere. The tissue of the sclera contains very few cells. It consists chiefly 

 of the lifeless fibers, and its rate of living (metabolism) is so low that 

 it requires no direct blood supply. Nearly all of the blood vessels to be 

 seen in sections of the sclera are merely passing through it on their way 

 into or out of the chorioid coat. 



The layers of fibers in the cornea (Fig. 4b) are not so much felted 

 as in the sclera, but run more nearly parallel with less interchange of 

 fibers between layers. The cells between them are consequently more 

 definitely organized into layers also; but they are scattered very far apart 

 in a given layer. The substance of the healthy cornea is quite devoid of 

 blood vessels, which would interfere with transparency. At the same 

 time, it is so firm that the diffusion of liquids through it is much im- 

 peded. Its living cells, the corneal corpuscles, therefore join hands by 

 means of long, delicate threads of living protoplasm along which nutri- 

 ments and wastes may be transported to and from the blood vessels 

 surrounding the margin of the cornea. The avascularity of the cornea, 

 and evaporation from its surface, make it several degrees cooler than the 

 body as a whole, and the metabolism of the corneal cells is adjusted to 

 the lower temperature. 



The change in the character of the tissue, as one passes from the sclera 

 into the cornea, is a gradual one and the wide region of transition noted 

 marks the limbus (rim) of the cornea. A flange of scleral substance, the 

 scleral roll, (Fig. 5, sr) overlaps the edge of the cornea on its inner sur- 

 face so that the illusion of the cornea being set in the sclera, like a watch- 

 crystal in its bezel, is created. The two portions of the fibrous tunic are 

 not actually at all easily separable, but the limbus is the weakest region 



