RETINA. STRUCTURE. 499 



may be divided into large and small, the latter being situated in the 

 spaces between the former. Their periphery is connected with the 

 ciliary ligament, and is continuous with the middle and internal layer 

 of the choroid. The central border is free, and rests against the cir- 

 cumference of the lens. The anterior surface corresponds with the 

 uvea; the posterior receives the folds of the zonula ciliaris between 

 its processes, and thus establishes a connection between the choroid 

 and the third tunic of the eye. The ciliary processes are covered with 

 a thick layer of pigmentum nigrum, which is more abundant upon 

 them, and upon the anterior part of the choroid, than upon the pos- 

 terior. When the pigment is washed off, the processes are of a 

 whitish colour. 



THIRD TUNIC. The third tunic of the eye is the retina, which is 

 prolonged forwards to the lens by the zonula ciliaris. 



Dissection. If after the preceding dissection the choroid membrane 

 be carefully raised and removed, the eye being kept under water, the 

 retina may be seen very distinctly. 



The Retina is composed of three layers : 



External or Jacob's membrane, 

 Middle, Nervous membrane, 

 Internal., Vascular membrane. 



Jacobs membrane is extremely thin, and is seen as a flocculent film 

 when the eye is suspended in water. Examined by the microscope, 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 processes, where it terminates by an abrupt scalloped 

 margin. 



According to Treviranus, this layer is composed of cylindrical fibres, 

 which proceed from the optic nerve and bend abruptly inwards, near 

 their termination, to form the internal papillary layer, which lies in 

 contact with the hyaloid membrane ; each fibre constituting by its ex- 

 tremity 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 

 zonula ciliaris. The vascular layer forms distinct sheaths for the ner- 

 vous papillae, 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 ;* 



* Samuel Thomas Soemmering is celebrated for the beautiful and accurate 



