ANATOMY OF THE EYEBALL. 55 



der water, the retina is seen, in perfectly fresh specimens, in 

 the form of an exceedingly delicate and transparent mem- 

 brane covering the posterior portion of the vitreous humor. 

 A short time after death, it becomes slightly opaline. It 

 extends over the posterior portion of the eyeball to a 

 distance of about -^ of an inch behind the ciliary processes. 

 When torn from its anterior attachment, it presents a finely- 

 serrated edge, called the ora serrata. This edge adheres very 

 closely, by mutual interlacement of fibres, to the zone of 

 Zinn. In the middle of the membrane, its thickness is about 

 Y^J of an inch. It becomes thinner near the anterior mar- 

 gin, where it measures only about -g-J-^ of an inch. Its ex- 

 ternal surface is in contact with the choroid, and its internal, 

 with the hyaloid membrane of the vitreous humor. 



The optic nerve penetrates the retina about -J- of an inch 

 within and ^ of an inch below the antero-posterior axis of 

 the globe, presenting, at this point, a small, rounded eleva- 

 tion upon the internal surface of the membrane, perforated 

 in its centre for the passage of the central artery of the retina. 

 At from -fa to -J- of an inch external to the point of pene- 

 tration of the nerve, is an elliptic spot, its long diameter being 

 horizontal, about -J of an inch long and -^ of an inch broad, 

 called the yellow spot of Sommering, or the macula lutea. 

 In the centre of this spot, is a depression, called the f ovea cen- 

 tralis. This depression is exactly in the axis of vision. The 

 yellow spot exists only in man and the quadrumana. 



The layers of the retina which present the greatest physi- 

 ological interest are the external layer, formed of rods and 

 cones, the layer of nerve-cells, and the filaments which con 

 nect the rods and cones with the cells. These are the only 

 anatomical elements of the retina, as far as we know, that are 

 directly concerned in the reception of optical impressions, and 

 they will be described rather minutely, while the intermediate 

 layers will be considered more briefly. 



Most modern anatomists recognize eight distinct layers in 

 the retina, as follows : 



