450 RETINA STRUCTURE. 



The Retina is composed of three layers : 



External, or Jacob's membrane, 

 Middle, Nervous membrane, 

 Internal, Vascular membrane. 



Jacob's membrane is extremely thin, and is seen as a mere film when 

 the freshly dissected eye is suspended in water. Examined by the micro- 

 scope, it is found to be composed of cells having a tesselated arrangement. 

 Dr. Jacob considers it to be a serous membrane. 



The Nervous membrane is the expansion of the optic nerve, and forms 

 a thin semi-transparent bluish-white layer, which envelopes the vitreous 

 humour, and extends forwards to the commencement of the ciliary pro- 

 cesses, w r here it terminates by an abrupt scalloped margin. According to 

 Treviranus, this layer is composed of cylindrical fibres, which proceed 

 from the optic nerve, and, near their termination, bend abruptly inwards, 

 to form the internal papillary layer, which lies in contact with the hyaloid 

 membrane ; each fibre constituting by its extremity a distinct papilla. 



The Vascular membrane consists of the ramifications of a minute artery, 

 the arteria centralis retinae, and its accompanying vein ; the artery pierces 

 the optic nerve, and enters the globe of the eye through the porus opticus, 

 in the centre of the lamina cribrosa. This artery may be seen very dis- 

 tinctly by making a transverse section of the eyeball. Its branches are 

 continued anteriorly into the zonula ciliaris. The vascular layer forms 

 distinct sheaths for the nervous papillae, which constitute the inner surface 

 of the retina. 



In the centre of the posterior part of the globe of the eye the retina pre- 

 sents a circular spot, which is called the foramen of Soemmering ;* it is 

 surrounded by a yellow 7 halo, the limbus luteus, and is frequently obscured 

 by an elliptical fold of the retina, which has been regarded as a normal 

 condition of the membrane. The term foramen is misapplied to this spot, 

 for the vascular layer and the membrana Jacobi are continued across it ; 

 the nervous substance alone appearing to be deficient. It exists only in 

 animals having the axes of the eyeballs parallel with each other, as man, 

 quadrumana, and some saurian reptiles, and is said to give passage to a 

 small lymphatic vessel. 



The Zonula ciliaris (zonula of Zinn)f is a thin vascular layer, which 

 connects the anterior margin of the retina with the anterior surface of the 

 lens near its circumference. It presents upon its surface a number of 

 small folds corresponding with the ciliary processes, between which they 

 are received. These processes are arranged in the form of rays around 

 the lens, and the spaces between them are stained by the pigmentum 

 nigrum of the ciliary processes. They derive their vessels from the vas- 

 cular layer of the retina. The under surface of the zonula is in contact 

 with the hyaloid membrane, and around the lens forms the anterior fluted 

 wall of the caruil of Petit. 



* Samuel Thomas Soemmering is celebrated for the beautiful and accurate plates 

 which accompany his works. The account " De Foramine Centrali Retinae Humana?, 

 Limbo Luteo cincto," was published in 1779, in the Cofrimentationes Soc. Reg. Scienl. 

 Gottingensis. 



} John Gottfried Zinn, professor of Anatomy in Gottingen; his "Descriptio Anatomicn 

 Oculi Humani" was published in 1755, with excellent plates. It was republished by 

 Wrisberg iu 1780. 



