THE RETINA 



745 



trunks, the venae vorticosae, which are formed by the convergence in whorls of 

 numerous veins, coming not only from the chorioid, but also from the ciUary body 

 and iris. The venae vorticosae perforate the sclera about at the equator and join 

 the veins of the ocular muscles. 



The nerves come from the long and short ciliary nerves. They form a plexus 

 in the lamina suprachorioidea, which contains ganglion cells, and sends numerous 

 non-medullated fibers chiefly to the blood-vessels of the chorioid. At the ciliary 

 muscle a second plexus (P. gangliosus ciliaris) is formed, which supplies the muscle 

 and sends fibers to the iris. The sphincter pupillae is supplied by fibers derived 

 from the oculomotor nerve, while the dilatator pupillae is innervated by the sym- 

 pathetic. 



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 



Fig. 560. — Inner Surface of Anterior Part of 

 Eyeball of Horse (Equatorial Section). 

 1, Sclera; 2, chorioidea; 3, retina (drawn anay 

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

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

 Ellenberger, in Leisering's Atlas.) 



Fig. 561. — Fundus Oculi, seen on Equatorial Sec- 

 tion OF Eyeb.\ll of Horse. 

 1, Sclera; S, chorioidea; S, retina (loosened); 

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

 Ellenberger, in Leisering's .\tlas.) 



three parts. The large posterior part, which alone contains the nervous elements, 

 including the s]iecial ncuro-epithelium and the optic nerve-fibers, is termed the 

 pars optica retinae. It extends forward to the ciliary body, where it terminates at 

 an irregular line called the era serrata. 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. In the pars ciliaris 

 retinae the inner stratum is non-pigmented, while the outer layer is a direct con- 

 tinuation of the stratum pigmenti of the pars optica. In the pars iridica retinae 

 both are pigmented. In the dead subject the pars optica is an opaque, gray, soft 

 membrane which can be stripped of the chorioid, leaving most of its outer pigmented 

 layer on the latter. During life it is transparent, except as to its pigmented epi- 

 thelium, 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. below the horizontal meridian and 3 to 4 mm. ex- 

 ternal to the vertical meridian. The central part of the papilla is slightly depressed 

 (Excavatio papillae n. optici). 



