THE SCLERA 569 



almost impossible for the microscope to discover any of the struc- 

 ture of the cornea. After death the cornea becomes opaque and its 

 elements are then easily distinguished. 



Vascular and Nerve Supply. The cornea itself is an absolutely 

 non-vascular tissue, having neither blood nor true lymphatic vessels. 

 It is, however, well supplied with nerve fibres, derived from the 

 ciliary nerves, which form an annular plexus in the sclera about 

 the margin of the cornea, from which point bundles of naked axis- 

 cylinders pass into the corneal substance to form a basal plexus 

 near the anterior homogeneous membrane. From this latter 

 plexus, fibres are distributed to the corneal substance and to a 

 subepithelial plexus, anterior to Bowman's membrane, whence ter- 

 minal fibrils penetrate the anterior epithelium. 



THE SCLERA. The sclera (sclerotic coat) is a firm opaque con- 

 nective tissue membrane which forms the outermost layer of the 

 posterior segment of the eyeball. It consists of two layers, the 

 thick, firm, substantia propria, and the very thin, delicate, lamina 

 fusca. 



By reflected light the sclera of the adult is of a lustrous white 

 color. In the child it has a faint bluish tint, due to the presence 

 of pigment in the deeper layers of the child's eye which shows 

 indistinctly through the relatively clear superficial tissues. The 

 anterior portion of the sclera is covered by the conjunctiva and is 

 familiarly known as the " white of the eye/' 



That portion of the sclera which is posterior to the ocular 

 equator is covered by the visceral layer of the capsule of Tenon 

 except at the insertions of the straight and oblique muscles. The 

 tendons of these muscles pierce the capsule and are obliquely in- 

 serted into the surface of the sclera in a line nearly correspond- 

 ing to the equator of the eye. The tendon bundles of the muscles 

 are directly continuous with the fibrous bundles which compose 

 the sclera. 



The Substantia Propria. The white fibrous tissue of the sclera 

 is disposed in bundles which are arranged along meridional and 

 equatorial lines ; they interlace with one another to form a dense 

 network. A few elastic fibres are interspersed among the bundles 

 of this network. 



The Lamina Fusca. The inner surface of the sclera presents a 

 fine gauzy membrane which can be readily detached by teasing. 

 This is the lamina fusca sclerae. It consists of delicate interlacing 

 fibrous bundles and numerous piginented connective tissue cells. 



