684 THE ORGANS OF SENSE. 
of the anterior pole. The anterior surface of the cornea is almost, but not quite, 
circular, measuring 11 mi. vertically and 11°9 mm. transversely, while its pos- 
terior surface is circular and has a diameter of 13 mm. Its periphery is overlapped 
by the tissue of the sclera as the glass of a watch is overlapped by the metal rim, 
: with, however, this essential dif- 
‘sun ference, that the tissue of the 
> epithelium : ; c : 5 
1 aaberibe cornea 1s directly continuous 
==> elastic with that of the sclera. 
\Jamina mee 8 x 
| The cornea consists, from be- 
fore backwards, of the following 
| strata, viz. (Fig. 494) :— 
l. A layer of stratified 
epithelium. 
; ; 2. An anterior elastic 
Substantia : 
propria, in lamina. 
camneal COE 3. The substantia propria. 
Dace 4. A posterior elastic lamina. 
see JO 2 ~ . . 
spindle- d. A layer of endothelium. 
shaped 01 ° . 
Sec 1. The layer of stratified epi- 
thelium (epithelium cornez) is. 
continuous with that which 
| covers the free surface of the 
| conjunctiva and consists of six 
or eight strata of nucleated cells. 
yaaa Deepest of all is a single layer 
elasticlamina Of perpendicularly-arranged 
; _ Endothelium Golumnar cells, the flattened and. 
Fic. 494.—VERTICAL SECTION OF CORNEA (magnified). : : ae ee 3 ; 
often-expanded bases of which 
rest on the anterior elastic lamina, while their opposite ends are rounded and contain 
the nuclei. Superficial to this layer are three or four strata of nucleated, polygonal 
cells, the majority of which exhibit finger-like processes which join with the corre- 
sponding processes of neighbouring cells. The more superficial layers assume the 
form of nucleated squames. The entire thickness of this stratified epithelium is. 
about 45 p at the centre and about 80 » at the periphery of the cornea. 
The anterior elastic lamina (lamina elastica anterior, Bowmani) is from 
19-20 p thick, and may be regarded merely as a differentiation of the outer part of 
the substantia propria from which it is with difficulty separated. It is not stained 
yellow by picrocarmine, thus differing from true elastic tissue, and its degree of 
development varies in different animals. . 
5. The substantia propria presents, in a fresh condition, a homogeneous appear- 
ance ; but, with the assistance of reagents, it is seen to consist of modified con- 
nective tissue, with some few elastic fibres. An amorphous interstitial substance 
binds the fibres into bundles, and, in turn, cements the bundles into lamellae, which 
are directly continuous with the fibrous tissue of the sclera. The fibres of any 
one lamella cross those of adjacent lamelle almost at right angles, while the 
superunposed lamelle are joined by sutural fibres and by amorphous substance. 
Between the lamelle are found the cell spaces or lacune of the cornea—irregu- 
larly stellate in shape, and communicating freely with each other by means of fine 
canaliculi. The corneal cells or corpuscles are contained in these lacune, without, 
however, completely filling them, the remainder of the cavities being occupied by 
lymph. These cells are nucleated, flattened, and star-like, and their branched pro- 
cesses join those of neighbouring cells in the canalicuh. Migratory or lymph cells 
are also found in the cell spaces. 
In old age a grayish opaque ring, 1} to 2 mm. in breadth, is frequently seen near the peri- 
phery of the « cornea ; it is termed the arcus senilis, and results from a deposit of fat granules in 
the lamella and corneal corpuscles. 
4. The posterior elastic lamina (lamina elastica posterior, Demoursi, Descemeti) 
is a Clear, structureless membrane, covering the posterior aspect of the substantia 
