The =fibrous tunic= (Tunica fibrosa oculi) is the external coat and is
composed of a posterior opaque part, the sclera, and a transparent
anterior part, the cornea.
The =sclera=, popularly termed the “white of the eye,” is a dense
fibrous membrane which forms about four-fifths of the fibrous tunic.
Thickest in the vicinity of the posterior pole (ca. 2 mm.), it thins at
the equator (ca. 0.4 mm.), and increases in thickness toward the
junction with the cornea (ca. 1.3 mm.). It is in general white, but may
have a bluish tinge in its thinnest parts. Its external surface
furnishes insertion to the ocular muscles and is covered by the
conjunctiva scleræ in its anterior part. The episcleral tissue, which is
richly supplied with vessels and nerves, attaches the conjunctiva to the
sclera; it is abundant and loosely meshed except at the junction with
the cornea. The inner surface is attached to the chorioid coat by a
layer of delicate, pigmented, connective tissue, the =lamina 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 scleræ=, 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
scleræ=.
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 venæ vorticosæ and ciliary veins. The lymphatics are
represented by intercommunicating cell spaces. The nerves are derived
from the ciliary nerves.
[Illustration: