682 THE ORGANS OF SENSE. 
(Fig. 493). The anterior or corneal segment, in consequence of its shorter radius, 
projects as a dome in front of the scleral portion, the union of the two parts 
being indicated externally by a slight groove, the sulcus scleree. The central 
See 7 Cornea 
— 
H oS 
i Anterior chamber 
Canal of'Schlemm = 1 i SGN 
t 
8, aa \ Tris 
Sie Sa 
A == st WH Posterior chamber 
Suspensory ligament 224 aaa ete NS Ciliary process 
Lens LP —— BSS conal of Petit 
Tendon of 
internal 
Tendon of F rectus 
external ny H 
rectus 4 H 
St \ 
fe Y 
fa ¥ 
il ¥ 
12a \ ; 
= \ \ Canal of 
eae Stilling 
Equator. --Equator 
Vitreous 
Optic axis 
Fic. 493, -D1aGRaM OF A HORIZONTAL SECTION THROUGH LEFT EYEBALL AND Optic NERVE 
(four times enlarged), 
points of the anterior and posterior curvatures of the eyeball constitute 
respectively its anterior and posterior poles, while a straight line joining the two 
poles is termed its sagittal axis (axis optica). The axes of the two eyeballs are 
almost parallel, diverging only slightly in front; but the axes of the optic nerves 
converge behind, and, if prolonged backwards, would meet in the region of the 
dorsum sellee of the sphenoid. An imaginary line encircling the globe midway 
between its two poles is named its equator, and meridional lines (meridiani) may be 
drawn from pole to pole at right angles to the equator. Its sagittal and transverse 
diameters are nearly equal—about 24 mm. ; its vertical diameter is about 23°5 mm. 
All three diameters are rather less in the female than in the male, but the size of 
the eyeball is fairly constant in the same sex. What are popularly described as 
large eyes owe their apparent increase in size to a greater prominence of the globe 
and to a wider fissure between the eyelids. 
At birth the eyeball is nearly spherical and has a diameter of about 17-5 mm. By the 
age of puberty this has increased to 20 or 21 mm., after which it rapidly reaches its adult 
size. 
The eyeball (Fig. 493) consists of three concentric tunics or coats, contained 
within which are three transparent refracting media. The three tunics are: (1) an 
outer fibrous coat, the sclero-cornea, consisting of an opaque posterior part, the 
sclera, and a transparent anterior portion, the cornea; (2) an intermediate 
vascular, pigmented, and partly muscular tunic, the tunica vasculosa oculi, com- 
prising from behind forward the chorioid, the ciliary body, and the iris; (3) an 
internal nervous tunic, the retina. The three refracting media are named, 
