THE CORNEA 



1059 



rated by the entrance of the optic nerve, and the opening in the sclera, only 

 partially bridged across by fibres from the inner layers, forms the lamina 

 cribrosa. 



The fibre-bundles composing the sclera are arranged more irregularly than in the cornea, 

 and run mainly in two directions, viz., antero-posteriorly and circularly; the circular fibres 

 are particularly well developed just behind the sulcus. It is thickest (about 1 mm.) posteriorly, 

 where it is strengthened chiefly by the outer sheath of the optic nerve, and partly also by the 

 tissue surrounding the ciliary vessels and nerves. It becomes gradually thinner as it passes 

 forward, up to the line of insertion of the recti muscles, where it is .3 mm. thick. In front of 

 that Une it is again reinforced by their tendinous fibres becoming incorporated with it and its 

 thickness increases to .6 mm. In children the sclera is often so thin as to allow the underlying 

 chorioidal pigment to show through, its colour then appearing bluish white. In the aged, again, 

 it is sometimes yellowish. It always contains a few pigment cells, but these are in the deep 

 layer termed the lamina fusca, and only become visible externally where the sclera is pierced 

 by vessels and nerves going to the chorioid. It is almost non- vascular, but quite at its anterior 

 end a large venous sinus [sinus venosus sclerse; canalis Schlemmi (Lauthi)], (canal of Schlemm) 

 runs in its deeper layers circularly around the cornea. Just in front of this sinus, at the corneal 

 limbus, the sclera merges into the cornea, its inner layers changing first, and finally the outer 

 ones. 



Fig. 803. — Portion of Fig. 802, Enlarged. 



Anterior surface of lens 

 Crystalline lens 



Sulcus scleras 



( Lig. pectinatum iridis 

 ( /Posterior chamber 



/ Sinus venosus scleras 

 / /' Scleral conjunctiva 



Anterior chamber 



Angulus iridis 



■"Circulus arteriosus major 



— Ciliary muscle, circular fibres 

 ^ Ciliary processes 

 Zonular fibres 



Ora serrata 



Insertion of 

 - tendon of 

 rectus lateralis 



The cornea forms the anterior sixth of the ej'eball. It is thickest 'at 

 its periphery (1.1 mm.) and becomes gradually thinner toward its centre 

 (0.8 mm.); 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 sclera. 



In the cornea proper, fibre-bundles are arranged so as to form a series of superposed lamellse, 

 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 communi- 

 cate 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 exception of a rich 

 plexus at its extreme periphery, on which its nutrition is ultimately dependent. The sinus 

 venosus of Schlemm is an important channel for the return of blood and also of fluid which 

 transudes into it from the anterior chamber. It consists of a network of venous spaces, formed 

 of a principal vessel accompanied by several smaller ones, which unite with it and with one 

 another in a plexiform manner. They commence indirectly with the spaces of the angle of the 

 iris and they are in direct communication with the anterior ciliary veins. 



