THE EETINA. 



815 



Iris 



Ciliary process. 



Sclera 



Chorioid 



Retina 



- Lens 



Ciliary processes 



Pars ciliaris 

 "retinae 



Ora serrata 

 Retina 



FIG. 686. A SEGMENT OF THE BULBUS OCULI SHOWING THE ORA 

 SERRATA. 



the retina appears to end, a short distance behind the ciliary body, in a wavy 

 border, the ora serrata (Fig. 686). There its nervous elements cease and the mem- 

 brane becomes suddenly 'thinned, but a delicate continuation of it is prolonged 

 over the posterior aspect 



of the ciliary body and Cornea 



iris. This continuation 

 consists of the pig- 

 niented layer, together 

 with a layer of columnar 

 epithelium, and con- 

 stitutes the pars ciliaris 

 retinae and pars iridica 

 retinae, already referred 

 to (p. 813). The portion 

 behind the ora serrata 

 is termed the pars optica 

 retinae, and its thickness 

 gradually diminishes 

 from 04 mm. near the 

 entrance of the optic nerve, to 0*1 mm. at the ora serrata. It presents, at the 

 posterior pole of the eye, and therefore directly in the optic axis, a small, oval 

 yellowish spot, the macula lutea. The greatest or transverse diameter of the 

 macula measures from 2-3 mm. ; its central part is depressed and is named the 

 fovea centralis. About 3 mm. to the nasal side and slightly below the level of the 

 posterior pole is a whitish, circular disc, the optic disc, which corresponds with the 

 entrance of the op tic -nerve, and has a diameter of about 1-5 mm. The circum- 

 ference of the optic disc is slightly raised and is named the papilla nervi optici, 

 while its depressed central portion is termed the excavatio papillae nervi optici 

 (O.T. optic cup). The optic disc consists merely of nerve-fibres, the other layers 



of the retina being absent, and it constitutes the 

 " blind spot." 



The nervous layer of the retina is transparent 

 during life, but becomes opaque and of a grayish 

 colour soon after death. If an animal is kept 

 in the dark before the removal of it's eyeball, the 

 retina presents a purple tinge, due to the presence 

 of a colouring matter named rhodopsin or visual 

 purple, which is rapidly bleached on exposure to 

 sunlight. This colouring matter is absent from 

 the macula lutea, and absent also over a narrow 

 zone, 3-4 mm. in width, near the ora serrata. 



(Stratum 



/pigmenti 



Structure of the Retina (Figs. 687, 688, 

 689). The nervous elements of the retina are sup- 

 ported by non-nervous or sustentacular fibres, and 

 are arranged in seven layers, to which must be added 

 the stratum pigmenti. 



The layers from within outwards, i.e. from 

 vitreous body to chorioid, are : 



1. Stratum opticum or layer of nerve-fibres. 



2. Ganglionic or nerve-cell layer. 



v. 3. Inner molecular or inner plexifc-rm layer. 



4. Inner nuclear layer or layer of inner granules. 



5. Outer molecular or outer plexiform layer. 



6. Outer nuclear layer or layer of outer granules. 



7. Layer of rods and cones. 



8. Stratum pigmenti. 



1. Stratum opticum or layer of nerve-fibres. Most of the fibres of this 

 stratum are centripetal, and are direct continuations of the axons of the cells in the 



Gangli- 

 onic 

 layer 



Stratum 

 f opticum 



Membrana limitans interna 



FIG. 687. DIAGRAMMATIC SECTION OF THE 

 HUMAN RETINA (modified from Schultze). 



