j 
THE RETINA. 689 
The non-striped muscular fibres are arranged in two sets: («) circular, (+) radial. 
The circular fibres form a band around the pupillary aperture, by the rae of 
which its size is lessened, and -hence it is termed the m. sphincter pupille. The 
radial fibres extend outwards from the sphincter to the cilary margin and constitute 
the m. dilatator pupille. Many anatomists regard the radial fibres, in man and 
most mammals, as being elastic and not muscular. In animals, where the radial 
fibres are muscular, the degree of their development varies considerably ; they are 
feebly marked in the rabbit, but well developed in the bird, and still more so in the 
otter. 
The nerves of the chorioid and iris are derived from the long and _ short 
ciliary nerves. The former, two or three in number, are branches of the nasal 
nerve; the latter, varying from eight to fourteen, are derived from the ciliary 
ganglion. Piercing the sclera around the optic entrance the nerves traverse the 
perichorioidal lymph space, where they form a plexus, rich in nerve-cells, from which 
filaments are supplied to the blood-vessels of the chorioid. In front of the ciliary 
muscle a second plexus, also rich in nerve-cells, is formed, which supphes the muscle 
itself and sends filaments into the iris as far as its pupillary margin, for the supply 
of its muscular fibres and blood-vessels. The sphincter pupille is supplied by the 
third cranial nerve, whilst filaments from the sympathetic are distributed to the 
dilatator pupillee. 
THE RETINA. 
The retina, or nervous tunic of the eyeball, is a soft, delicate membrane, in which 
the fibres of the optic nerve are spread out. It consists of two strata, viz.: (@) an 
outer, pigmented layer (stratum pigmenti), attached to the chorioid; and ()) an 
inner nervous lamina, the retina proper, in contact with the hyaloid membrane of 
the vitreous body, but only attached to it at the optic entrance and in the region 
of the ciliary processes. Expanding from the entrance of the optic nerve the 
retina appears to end, a short distance behind the cilary body, in a wavy border, 
the ora serrata. Here its nervous elements cease and the membrane becomes 
suddenly thinned, but a delicate continuation of it is prolonged over the posterior 
aspect of the ciliary body and iris. This continuation consists of the stratum pig- 
menti, together with a layer of columnar epithelium, and constitutes the pars ciliaris 
retine and pars iridica retine, already referred to (pp. 687 and 688). The portion 
behind the ora serrata is ject the “physiological retina” or pars optica retine, 
and its thickness gradually diminishes from 0:-4mm., near the optic entrance, to 
0-1 mm. at the ora serrata. Viewed from the front it presents, at the posterior 
pole of the eyeball, and therefore directly in the axis of the globe, a small yellowish 
spot, the macula lutea. Somewhat oval in shape, the greatest or transverse 
diameter of the macula measures from 2-3 mm.; its central part is depressed and 
named the fovea centralis. About 3 mm. to the nasal side of the posterior pole 
and about 1 mm. below its level is a whitish, circular disc, the optic disc, or porus 
opticus, which corresponds with the entrance of the optic nerve and has a diameter 
of about 155 mm. The circumference of the optic disc is slightly raised and is 
named the colliculus nervi optici, while its depressed central portion is termed the 
optic cup, or excavatio papill nervi optici. The optic disc consists merely of nerve 
fibres, the other layers of the retina being absent, and constitutes the “ blind spot ” 
of physiologists. 
The nervous layer of the retina is transparent during life, but becomes opaque 
and of a greyish colour soon after death. If an animal is kept in the dark before 
the removal of its eyeball, the retina presents a purple tinge, due to the presence of 
a colouring matter named rhodopsin or visual purple, which, however, becomes 
rapidly bleached on exposure to sunlight. This colouring matter is absent from 
the macula lutea, and also over a narrow zone, 3-4mm. in width, near the ora 
serrata. 
Structure of the Retina (Figs. 499, 500, 501).—The nervous elements of the 
retina are supported by non-nervous or sustentacular fibres, and are arranged in seven 
layers, to which must be added the stratum pigmenti. 
44 
