SCLERA—CORNEA. 683 
from before backwards, the aqueous humour, the crystalline lens, and the vitreous 
body. 
SCLERA. 
The sclera, sclerotic coat, or white of the eye, is a firm, opaque membrane, which 
forms something like the posterior five-sixths of the outer tunic. Thickest 
posteriorly (about 1 mm.), it thins at the equator to 0-4 mm.-0°5 mm., and again 
increases to 0°6 mm. near the sulcus sclere. In the child it is thinner than in the 
adult and presents a bluish appearance, caused by the pigment of the chorioid 
shining through it, while in old age it assumes a yellowish tinge. Its outer surface 
is covered by a layer of endothelium and is in contact with the capsule of Tenon— 
a lymph space, the suprascleral lymphatic space, only intervening. In front of the 
equator it is roughened by the attachment of the tendons of the ocular muscles, 
while its anterior part is covered by mucous membrane, the conjunctiva. Its deep 
surface presents a brownish colour and is loosely attached to the chorioid, except at 
the optic entrance and in the neighbourhood of the sulcus scleree. It is pierced behind 
by the optic nerve, the entrance for which is funnel-shaped, wide behind and narrow 
in front, and is situated 1 mm. below and 3 mm. to the nasal side of the posterior 
pole of the eyeball. The fibrous sheath of the nerve blends with the outer part of 
the sclera, while the nerve bundles pass through a series of orifices; this perforated 
portion is named the lamina cribrosa sclere. Around the entrance of the optic nerve 
are some fifteen or twenty small apertures for the passage of the ciliary nerves and 
short ciliary arteries. The two long posterior ciliary arteries pierce it, one on 
either side, some little distance from the optic entrance; while a little behind the 
equator are four openings, two above and two below, for the exit of veins, called 
vene vorticose. Near the sulcus scler it is perforated by the anterior ciliary 
arteries. Its inner surface is lined by flattened endothelial cells; and between it 
and the chorioid is an extensive lymph space, the spatium perichorioideale, which - 
is traversed by the ciliary nerves and arteries just mentioned, and by an irregular 
meshwork of fine, pigmented, connective tissue, the lamina fusca, which loosely 
attaches the sclera to the chorioid. At the corneo-scleral junction the fibrous 
tissue of the sclera passes continuously into that of the cornea, and in the deeper 
part of this junction there is a circular canal, the sinus venosus sclere, or canal of 
Schlemm, which communicates externally with the scleral veins, and internally, 
through numerous small openings, with the anterior chamber of the eyeball. 
The sclera consists of bundles of white fibrous tissue, together with some fine 
elastic fibres, the bundles forming equatorial and meridional layers, which inter- 
lace with each other. Numerous spaces containing connective tissue cells and 
migratory cells exist between the fibres. Pigmented cells are plentiful in the 
lamina fusca, and a few are also found in the tissue of the sclera, near the optic 
entrance and in the region of the corneo-scleral junction. The sclera receives 
its blood-supply from the short posterior ciliary and anterior ciliary arteries, while 
its veins open into the ven vorticose and anterior ciliary veins. The cell spaces 
play the part of lymphatics, and communicate with the perichorioidal and supra- 
scleral lymph spaces. Its nerves are derived from the ciliary nerves, which, after 
losing their medullary sheath, pass between the fibrous bundles; their exact mode 
of ending is, however, not accurately known. 
CORNEA. 
The cornea forms, the anterior sixth of the outer tunic and is transparent, in 
order to admit light into the interior of the eyeball; its index of refraction is 
from 1°33 to 1:35. Its anterior surface (facies anterior) is covered by a stratified 
epithelium, continuous with that which lines the conjunctiva ; its posterior surface 
(facies posterior) is directed towards the anterior chamber of the eyeball and is in 
contact with the aqueous humour. Its degree of curvature varies in different 
individuals; it is always greater in youth than in old age, and is, as a rule, 
slightly greater in the vertical than in the horizontal plane. It diminishes also 
from its centre to its periphery, and is less on the nasal than on the temporal side 
