8638 THE SENSE ORGANS AND SKIN OF THE HORSE 
THE RETINA 
The retina or nervous tunic of the eyeball is a delicate membrane which extends 
from the entrance of the optic nerve to the margin of the pupil. It consists of 
three parts. The large posterior part, which alone contains the nervous elements, 
including the special neuro-epithelium and the optic nerve-fibers, is termed the 
pars optica retine. It extends forward to the ciliary body, where it terminates at 
an almost regular circular line called the ora ciliaris retine.! Here the retina 
rapidly loses its nervous elements, becomes much thinner, and is continued over the 
ciliary body and the posterior surface of the iris by two layers of epithelial cells as 
the pars ciliaris retine; the inner stratum is non-pigmented, while the outer layer 
is a direct continuation of the stratum pigmenti of the pars optica. The pars 
iridica retine is a layer of pigmented cells which covers the posterior surface of the 
iris. In the dead subject the pars optica is an opaque, gray, soft membrane which 
Fic. 697.—INNER SURFACE OF ANTERIOR PART OF Fic. 698.—Funpwus Ocul, SEEN ON EQUATORIAL SEC- 
EyYeBALL OF Horse (EQUATORIAL SECTION). TION OF EYEBALL OF HORSE. 
1, Selera; 2, chorioidea; 3, retina (drawn away 1, Seclera; 2, chorioidea; 3, retina (loosened); 
from chorioidea); 4, ciliary processes; 5, erystalline 4, tapetum; 4, optic papilla; 6, optic nerve. (After 
lens, through which the pupil (6) is seen (After Ellenberger, in Leisering’s Atlas.) 
Ellenberger, in Leisering’s Atlas.) 
can be stripped off the chorioid, leaving most of its outer pigmented layer on the 
latter. During life it is transparent, except as to its pigmented epithelium, and the 
reddish appearance of the fundus as viewed by the ophthalmoscope is caused by 
the blood in the network of the chorioeapillaris. The entrance of the optic nerve 
forms a sharply defined, oval, light area, the optic papilla (Papilla nervi optici), 
situated about 15 mm. ventral to the horizontal meridian and 3 to 4 mm. lateral 
to the vertical meridian. The central part of the papilla is slightly depressed 
(Excavatio papillze n. optic). 
The transverse diameter of the papilla is about 6 to 7 mm., and the vertical about 4 to 5 mm. 
It is commonly situated a little below the margin of the tapetum, but the latter may extend down 
somewhat on either side of the papilla. The lower margin is often indented a little. In inspee- 
tion of the fundus with the ophthalmoscope numerous fine branches of the arteria centralis retin 
are seen radiating from the periphery of the papilla. 
The optic nerve fibers converge from all parts of the pars optica to the papilla, 
where they collect into bundles which traverse the lamina cribrosa of the chorioidea 
and sclera, and constitute the optic nerve. The area centralis reting is a round 
spot, 2 to 3 mm. in diameter, situated dorso-lateral to the optic papilla; it corre- 
1 Jn man the line is finely serrated and is termed the ora serrata. 
