868 



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 retinae. It extends forward to the ciliary bodj^, where it terminates at 

 an almost regular circular line called the era ciliaris retina.' 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 retinae ; 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 retinae 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 



Fig, 697. — Inner Surface of Anterior Part of 

 Eyeball of Horse (Equatorial .Sbctionj. 

 i, Sclera; 2, chorioidea; 5, retina {drawn away 

 from chorioidea); 4i ciliary processes; 5, crystalline 

 lens, through which the pupil {6) is seen. (After 

 Ellenberger, in Leisering's Atlas.) 



Fig. 698. — Fundus Oculi, Seen on Equatorial Sec- 

 tion OF Eyeball of Horse. 

 1, Sclera; 2, chorioidea; 3, retina (loosened); 

 4, tapetum; 5, optic papilla; 6, optic nerve. (After 

 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 choriocapillaris. 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 papillae n. optici). 



The transverse diameter of the papilla ia 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 inspec- 

 tion of the fundus mth the ophthalmoscope numerous fine branches of the arteria centralis retinae 

 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 retinae is a round 

 spot, 2 to 3 mm. in diameter, situated dorso-lateral to the optic papilla; it corre- 



1 In man the line is finely serrated and is termed the ora serrata. 



