THE FIBROUS TUNIC 



741 



fusca. The anterior border is oval, the long axis being transverse, and is continuous 

 with the cornea. The transition from the opaque scleral tissue to the transparent 

 corneal substance occurs in such manner that the sclera appears to form a groove 

 (Rima cornealis), into which the cornea fits somewhat as a watch-glass in the case. 

 Near the corneo-scleral junction there is a circular venous plexus, the plexus s. 

 sinus venosus scleras, formerly called the canal of Schlemm. The optic nerve 

 passes through the posterior part of the sclera a little below and external to the 

 posterior pole. The opening for the nerve is crossed by interlacing fibrous strands, 

 forming the lamina cribrosa sclerae. 



The sclera consists of interlacing bundles of white fibrous tissue, associated 

 with which there are a few elastic fibers. The bundles are arranged chiefly in 

 meridional and equatorial layers. The very limited blood-supply is derived from 

 the ciliary arteries, and the veins open into the venae vorticosie and ciliary veins. 



Sclera 

 / ^Reflection of conjiinctitJa 



Ci rami la iridis 



Optic nerve 



Optic papilla 

 Retrobulbar fat 



Rectus oculi inferior 



Cornea 



Lens 

 Anterior chamber 



Ciliary processes 



Chorioid 

 Fig. 557. — Vertical Section of Eyeball of Horse, .\bout ?. 

 The contour of the crystalhne lens i.s dotted. 



The lymphatics are represented by intercommunicating cell spaces. The nerves 

 are derived from the ciliary nerves. 



The cornea forms the anterior fifth of the fibrous tunic. It is transparent, 

 colorless, and non-vascular. A'iewed from in front it is oval in outline, the long 

 axis being transverse and the broad end internal; it appears more nearly circular 

 when viewed from behind. Its anterior surface (Facies anterior) is convex and is 

 more strongly curved than the sclera; its central part is termed the vertex corneae. 

 The posterior surface (Facies posterior) is concave; it forms the anterior boundary 

 of the anterior cham])er, and is in contact with the aqueous humor. The margin 

 (limbus corneae) joins the sclera; the latter overlaps the cornea more in front than 

 behind, and more above and below than at the sides, thus explaining the difTerence 

 in outline of the two surfaces. The cornea is thinnest at the vertex. 



The cornea consists, from before backward, of the following layers: (1) The 

 epithelium corneae is continuous with that of the conjunctiva sclerae, and is of the 

 stratified scjuamous type. (2) The lamina limitans anterior is merely a condensa^ 



