THE EXTERNAL COAT THE FIBROUS TUNIC 635 



the coat becoming progressively thinner toward the equator of the eye; 

 near its anterior margin it is again thickened by the tendinous insertions 

 of the extrinsic muscles. 



The sclera is chiefly supplied by branches from the posterior ciliary 

 arteries, which form a wide-meshed plexus in its substance, its vessels 

 anastomosing freely with those of the choroid coat. 



THE SCLEROCORNEAL JUNCTION 



The sclerocorneal junction (Fig. 543) is a narrow circular zone at 

 the margin of the cornea, where it is inserted into the sclera. Across 

 this narrow zone the fibrous bundles of the opaque sclera are continued 

 directly into the similar, though perfectly transparent, bundles of the 

 cornea! substance. 



The anterior or outer surface of this zone is covered by the ocu- 

 lar portion of the conjunctiva. Its epithelium is of the stratified squam- 

 ous variety and is continuous with the anterior epithelium of the cor- 

 nea. 



From the inner surface of this junctional zone the anterior extremi- 

 ties of the muscle fibers composing the ciliary muscle take their origin. 

 The fibers of this muscle intermingle with the marginal fibers of the 

 posterior homogeneous layer of the cornea to form the Ugamentum 

 pectinatum, which connects the sclerocorneal junction with the base of 

 the iris. This pectinate ligament is very much more highly developed in 

 certain animals, e.g., cow and horse, than in man. 



Toward the inner side of the scleral margin and near the border 

 of the cornea is the canal of Schlemm (sinus venosus sclerce}. This 

 is an annular venous channel (or network of channels), draining into 

 the anterior ciliary veins. Though venous in character it serves also 

 as a drainage channel for the lymphatic spaces of Fontana, which lie in 

 the lateral wall of the anterior chamber and between the fiber bundles 

 of the ligamentum pectinatum. The spaces of Fontana are true lym- 

 phatic spaces and are in communication with the anterior chamber of 

 the eye. 



Through the canal of Schlemm, the aqueous humor of the anterior 

 chamber is put into communication with the veins of the sclera, and 

 a system is thus formed by which the intra-ocular pressure is maintained 

 at normal. A blocking of the canal of Schlemm interferes with the 

 drainage of the anterior chamber, and is believed to produce the serious 

 pathologic condition of the eyeball, known as glaucoma, due to progres- 



