RETINA STRUCTURE. 443 



External or Jacob's membrane, 

 Middle, Nervous membrane, 

 Internal, Vascular membrane. 



Jacob's membrane is extremely thin, and is seen as a flocculent 

 film when the eye is suspended in water. Examined by the micro- 

 scope, it is seen to be composed of granules 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 processes, where it terminates in an abrupt scalloped 

 margin. 



This layer has been observed by Treviranus to be composed of 

 cylindrical fibres, which proceed from the optic nerve and bend 

 abruptly inwards, near their termination, to form the internal papil- 

 lary 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 distinctly by making a transverse section 

 of the eyeball. Its branches are 

 continuous anteriorly with the zo- Fig. 138 -* 



nula ciliaris. 



This vascular layer forms distinct 

 sheaths for the nervous papilla?, 

 which constitute the inner surface 

 of the retina. 



In the centre of the posterior part, 

 of the globe of the eye the retina 

 presents a circular spot, which is 

 called the foramen of Soemmering ;f 

 it is surrounded by a yellow halo, 

 the limbus luteus, and is frequently 

 obscured by an elliptical fold of the 

 retina, which, from its constancy of 

 appearance, has been regarded as a normal condition of the mem- 



* The posterior segment of the transverse section of the globe of the eye, seen from 

 within. 1. The divided edge of the three tunics. The membrane covering the whole 

 internal surface is the retina. 2. The entrance of the optic nerve with the arteria cen- 

 tralis retinae piercing its centre. 3, 3. The ramifications of the arteria centralis. 

 4. The foramen of Soemmering, in the centre of the axis of the eye ; the shade from 

 the sides of the section obscures the limbus luteus which surrounds it. .5. A fold of 

 the retina, which generally obscures the foramen of Soemmering after the eye lias been 

 opened. 



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

 which accompany his works. The account " De Foraminc Centrali Retina? Humanse, 

 Limbo Lutco cincto," was published in 1779, in the Commi'nlationts Soc. Reg. Scient. 



