448 THE EYE. 



cle, the mesial half of which forms the lens fibers by a longitudinal 

 growth of its cells, while the lateral portion forms the thin anterior 

 epithelial capsule of the lens. The epithelium of the ectoderm 

 external to the lens differentiates later into the external epithelium 

 of the cornea and conjunctiva, neither of which structures is at 

 this stage sharply defined from the remaining ectoderm. It is only 

 during the development of the eyelids that a distinct demarcation 

 is established. All the remaining portions of the eye, as the vitre- 

 ous body, the vascular tunic with the iris, the sclera with the 

 substantia propria of the cornea and the cells of Descemet's laye.r, 

 are products of the mesoderm. 



C TUNICA FIBROSA OCULL 



I. THE SCLERA. 



The sclera is the dense fibrous tissue covering of the eyeball, 

 and is directly continuous with the transparent cornea. At the poste- 

 rior mesial portion of the eyeball, the sclera is perforated for the en- 

 trance of the optic nerve, this region being known as the lamina 

 cribrosa. The sclera consists of bundles of connective-tissue fibers 

 arranged in equatorial and meridional layers. At the external 

 scleral sulcus, in the vicinity of the cornea, the arrangement of the 

 fibers is principally equatorial. The tendons of the ocular muscles 

 are continuous with the scleral fibers in such a manner that those 

 of the straight muscles fuse with the meridional fibers, while those 

 of the oblique muscles are continuous with the equatorial fibers. 

 In the sclera are many lymph-channels communicating with those 

 of the cornea. They are much coarser and more irregularly arranged 

 than those of the cornea, and in this respect simulate the lymph- 

 channels found in aponeuroses. Pigmentation is constantly present 

 at the corneal margin, in the vicinity of the optic nerve entrance, 

 and also on the surface next the choroid. The innermost pigment 

 layer of the sclera is lined by a layer of flattened endothelial cells, 

 and is regarded by some as a separate membrane, known as the 

 lamina fusca ; generally, however, it is regarded as forming a part 

 of the outermost layer of the choroid (lamina suprachoroidea). The 

 external surface of the sclera also presents a layer of flattened endo- 

 thelial cells, belonging to the capsule of Tenon. Anteriorly, the 

 mobile scleral conjunctiva is attached to the sclera by a loose con- 

 nective tissue containing elastic fibers. 



The cornea is inserted into the sclera very much as a watch- 

 crystal is fitted into its frame. At the sclerocorneal junction is 

 found an annular venous sinus, the canal of Schlemm, which may 

 appear as a single canal or as several canals separated by incom- 

 plete fibrous septa. Anteriorly and externally this canal is bounded 

 by the cornea and sclera ; internally, it is partly bounded by the 

 origin of the ciliary muscle. The sclera comprises, therefore, one- 



