THE FIBROUS TUNIC 863 
posterior opaque, scleral segment one of about 25 mm. The anterior segment 
therefore projects more strongly, and the junction of the two segments is marked 
externally by a broad, shallow groove, the sulcus sclera. The central points of 
the anterior and posterior curvatures of the eyeball are termed respectively the 
anterior and posterior poles (Polus anterior, posterior), and the line connecting the 
poles is the external optic axis (Axis oculi externa).! The angle of divergence of the 
optic axes is about 137 degrees. The equator (Aquator) is an imaginary line drawn 
around the eyeball midway between its poles, and meridians (Meridiani) are lines 
drawn around it through the poles. 
The average transverse diameter of the eyeball is about 5 em., the vertical about 4.5 cm., 
and the axial about 4.25 cm. The distance from the anterior pole to the point of entrance of the 
optic nerve is about 3 cm. 
The eyeball consists of three concentric tunics or coats, within which three 
refractive media are enclosed. 
THE Fiprous TuNIC 
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. 
1. The sclera*is a dense fibrous 
membrane which forms about four- Lens seen through cornea 
fifths of the fibrous tunic. Thickest Corneo-scleral + 
in the vicinity of the posterior pole junction 
(ca. 2 mm.), it thins at the equator ial. \ 4 
(ca. 0.4 mm.), and increases in thick- eyelid Nai A rr 9 
\ 
ness toward the junction with the 
cornea (ca. 1.3 mm.). It is in gen- edi a i. 
eral white, but may have a bluish Sas a 
tinge in its thinnest parts. Its ex- 4 
ternal surface furnishes insertion to ~ 
the ocular muscles and is covered by 
the conjunctiva sclere 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 ns 
loosely meshed except at the junc- y 
tion with the cornea. The inner 
surface is attached to the chorioid Fic. 693.—Lerr Eyepsatt or Horse, in sifu, AFTER REMOVAL 
coat by a layer of delicate, pig- JSS NUROESTAEGN ea LEBES 
Henied connective tissue, the ,,,, ¢/7stmris ao 14, swaatial poses 1, oi 
lamina fusca. The anterior bor- — Kinstler.) 
der, which is oval, the long axis 
being transverse, 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 some- 
what as a watch-glass in the case. Near the corneo-scleral junction there is a 
circular venous plexus, the plexus s. sinus venosus sclere.*? The optic nerve 
passes through the posterior part of the sclera a little below and lateral to the 
posterior pole. The opening for the nerve is crossed by interlacing fibrous strands, 
forming the lamina cribrosa sclere. The sclera consists of interlacing bundles of 
white fibrous tissue, associated with which there are a few elastic fibers. The 
-+ - Sclera 
/ 
~~ Cornea 
Pupil 
1 The term internal optic axis (Axis oculi externa) is applied to a coincident Jine from the 
posterior surface of the cornea to the anterior surface of the retina. 
? Also termed the sclerotic coat, and popularly known as the “white of the eye.” 
3 Formerly termed the canal of Schlemm. 
