RETINA STRUCTURE. 339 



seventh month, when it disappears, and the fibres of both 

 chambers mingle and pass, readily, the one into the other. 

 Cases are mentioned, however, where this membrane has 

 remained after birth, causing blindness. This membrane, 

 according to M. Cloquet, consists of two layers both serous 

 and formed the one in front of the iris, lining the ante- 

 rior chamber, and constituting the anterior layer, and the 

 other, of the one behind the iris, lining the posterior cham- 

 ber, and forming the posterior layer. 



Function. To act as the diaphragm of the telescope, by 

 its black pigment intercepting all the rays of light except 

 those passing through the pupil. 



Retina. The retina (Fig. 100) forms the third or inner- 

 most coat, constituting the membranous case of the eye. It 

 lines the choroid, and is A FlG m B 



in contact^ with 'the vit- 

 reous humor. It is the 

 most important of the 

 three membranes, as it 

 is the seat of vision, and 

 is the place where the 

 images of objects are 

 painted. 



Dissection. The eye being kept under water, carefully 

 remove the choroid, when the retina is distinctly seen. 



Structure. It consists essentially of the expansion of the 

 optic nerve, which forms a middle layer of nervous coat, 

 covered internally by a vascular layer, and externally by 

 a serous one, discovered by Mr. Jacobs. The nervous coat 

 presents a bluish white appearance, is soft, pulpy, easily 

 torn, and extends forward to the ciliary processes. The 

 internal layer, or vascular coat, is compared to the pia-ma- 

 ter, and consists of the minute branchings of the arteria 



V 



FIG. 100, A represents the Retina after the removal of the Choroid. a 

 The Retina. 6 Optic nerve, c Iris, d Vitreous humor, e Where the ret- 

 ina terminates. 



FIG. 100, B represents the central artery of the retina, a Yellow spot of 

 Soemerring. 6 Point of entrance of optic nerve, c Choroid coat, s Sclerotic coat. 



