846 ORGAAS OF SPECIAL SENSE 



by the outer sheath of the optie nerve, and i)artly also ])y tlie tissue surrounding 

 the cihary vessels and nerves. It becomes gradually thinner as it passes forwards, 

 up to the line of insertion of the recti muscles, in front of which line it is again 

 reinforced by their tendinous fibres becoming incorporated Avith it. In children 

 the sclerotic is often so thin as to allow the underlying choroidal pigment to show 

 through, appearing then of a bluish white. In the aged, again, it is sometimes 

 yellowish. It always contains a few pigment cells, but these are in the deej) layers, 

 and only become visible externally where the sclerotic is })ierced by vessels and 

 nerves going to the choroid. It is itself almost non-vascular, but quite at its 

 anterior end a large venous sinus (canal of Schlemm) runs in its dee])er layers 

 circularly around the cornea. Just in front of this sinus, at the corneal limbus, 

 the sclerotic merges into the cornea, its deep layers changing first, and tinally the 

 su})erticial ones. 



The cornea is thickest at its periphery, and becomes gradually thinner towards 

 its centre; the curvature of its posterior is consequently greater than that of 

 its anterior surface, but even the latter is more curved than the surface of the 

 sclerotic. In the cornea proper, fil^re-ljunclles are arranged so as to form a series 

 of superposed lamella?, each of which is connected here and there to the adjacent 

 ones by fibres passing from one to the other, so that they can only be torn apart 

 with difficulty. The corneal lymph-spaces communicate with one another by very 

 fine canals, and thus not only is a thorough lymph-circulation provided for, but 

 the protoplasm with which these spaces are partially occupied may be also 

 regarded as continuous throughout. It contains no blood-vessels, with the excep- 

 tion of a rich plexus at its extreme peripher}', on which its nutrition is ultimately 

 dependent. 



The most superficial part of the true cornea appears homogeneous, even when 

 highly magnified (Bowman's membrane), though there is reason to believe that 

 its structure only differs from that already described in the closeness of its fibrous 

 texture; the two parts are certainly connected by fine fibres. Anteriorh', the cornea 

 is covered hx an extension of the ocular conjunctiva, in the form of an epithelium 

 several la^-ers deep. Posteriorly, the cornea is lined b}' a firm, thin, glass-like layer 

 (membrane of Descemet, posterior elastic lamina), distinct from the corneal 

 tissue both anatomically and chemically. At the periphery this membrane breaks 

 up into a nuniV)er of fil)res. which mainly arch over to join the base of the iris 

 (ligamentum pectinatum iridis). The interstices between these fibres constitute 

 spaces (spaces of Fontana ) freely communicating with the aqueous chamber on the 

 one hand, and indirectly with the canal of Schlemm on the other. Descemet' s 

 mem1)rane is in turn lined hx a single layer of flat cells, which are continuous 

 peripherally Avith cells lining the spaces of Fontana and the anterior surface of the 

 iris. The cornea is richly supplied with nerves, particularly in its most superficial 

 layers. 



2. The dark, middle, or vascular coat of the eye, generally known as the 

 uveal tract, is formed ))y the iris, ciliary body, and choroid. It is closely applied 

 to the sclerotic, but actually joins it only at the anterior and posterior limits of their 

 course together, viz. at the scleral sulcus, and around the optic nerve entrance. In 

 front of the sulcus the middle coat no longer lines the outer, being separated from 

 it (i.e. the iris from the cornea) by a consideraWe space filled with fluid, called the 

 anterior aqueous chamber. The uveal tract has two openings in it; a larger one in 

 front, the pupil, and a smaller one behind, for the passage of the optic nerve. Its 

 structure is that of a pigmented connective tissue, supporting numerous blood- 

 vessels and containing many nerves and two deposits of smooth muscle-fibres. 



The choroid forms the posterior part of the uveal tract, and extends, with 

 slowly diminishing thickness, forwards as far as the ora serrata. Its outer and 

 inner surfaces are both formed by non-vascular layers; that covering the outer, the 

 membrana suprachoroidea, is jngmented, arranged in several fine loose lamella", 

 and has been seen in our dissection; that covering the inner surface is a thin, trans- 

 parent, homogeneous membrane, called the vitreous lamina of the choroid, or the 

 membrane of Bruch. The intervening choroidal stroma is very rich in blood- 

 vessels, which are of largest size next its outei' surface, and become progressively 

 smaller as we approach the vitreous lamina, next to which we find a layer of closely 



