THE NERVOUS TUNIC. 



347 



nerve entrance to the pupillary border. The functioning portion, or pars 

 optica retina, reaches as far forwards as an irregular wavy line, the ora 

 serrata ; anterior to this, the retina is represented by an atrophic portion, 

 consisting of a double layer of cells covering the ciliary body and the iris, 

 respectively the pars ciliaris and pars iridica retince. The pars optica 

 retinse is closely applied to the inner surface of the choroid and is in contact 

 with the hyaloid membrane investing the vitreous body. It gradually di- 



Cornea 



Greater arterial ring 

 Iris 



Lesser arterial ring 

 Ciliary process 



Canal of Schlemm 



^Cornea! loop 



Perforating branch 



Conjunctival vessels 



Anterior ciliary 

 vessels 



Sclera 

 Episcleral vessels 



Vena vorticosa 



Communication between Retinal vessels dwS&f- JD* 



choroidal and optic vessels y^/O/Wt Til Supplying choroid 



Central retinal vessels \ 



Short post. cil. art. 



Long post. cil. art. 

 Communicating twig 

 Inner sheath vessels 

 Outer sheath vessels 



-Communicating vessels 

 FIG. 396. Diagram illustrating circulation of eyeball. (Leber.) 



minishes in thickness from .4 mm. at the posterior pole to . i mm. near the 

 ora serrata. At the posterior pole of the eyeball, 3 mm. to the outer side 

 of the optic nerve entrance, the retina exhibits an oval area, the yellou> spot 

 or macula lutea ; the centre of the latter is marked by a small depression, 

 the fovea centralis, which corresponds to the region of sharpest vision. 



The retina is composed of nervous elements which are supported by a 

 specialized sustentacular tissue or neuroglia. Morphologically it must be 

 considered as composed of two lamellae, which correspond to the outer and 

 inner walls of the optic vesicle, the hollow outgrowth of the brain-sac from 



