902 



THE ORGANS OF SENSE. 



presents a clear oval nucleus, referred to above, which is sometimes situated at the 

 side of, sometimes altogether within, the fibre. In the outer nuclear layer the fibre 

 breaks up into fine lamellae, which form a fenestrated or sponge-like tissue, in 

 which the rod- and cone-granules are enclosed, and at the outer border of this 

 layer these lamellae unite along a definite line, forming the mernbrana limitans 

 externa. 



Macula Lutea and Fovea Centralis. The structure of the retina at the yellow 

 spot presents some modifications. In the macula lutea (1) the nerve-fibres are 



Pigmentary layer. 



Jacob's membrane. 



&- Membrana limitans externa. 



Outer nuclear layer. 



Outer molecular layer. 



Fibre of Milller. 



Inner nuclear layer. 



Inner molecular layer. 



Vesicular layer. 

 Fibrous layer. 



Membrana limitans interna. 

 FIG. 535. The layers of the retina (diagrammatic). (After Schultze.) 



wanting as a continuous layer ; (2) the vesicular layer consists of several strata of 

 cells, instead of a single layer; (3) in Jacob's membrane there are no rods, but 

 only cones, and these are longer and narrower than in other parts ; and (4) in the 

 outer nuclear layer there are only cone-fibres, which are very long and arranged in 

 curved lines. At the fovea centralis the only parts which exist are the cones of 

 Jacob's membrane, the outer nuclear layer, the cone-fibres of which are almost 

 horizontal in direction, and an exceedingly thin inner granular layer. The color 

 of the spot seems to imbue all the layers except Jacob's membrane ; it is of a rich 

 yellow, deepest toward the centre, and does not appear to consist of pigment-cells, 

 but simply a staining of the constituent parts. 



At the ora serrata the layers of the retina for the most part terminate abruptly, 

 and the radiating fibres of Muller, covered by the pigmentary layer, can be traced 

 forward, as the pars ciliaris, to the iris. The fibres of Muller here present the 

 appearance of columnar epithelial cells arranged in a single stratum. 



The arteria centralis retince and its accompanying vein pierce the optic nerve, 

 and enter the globe of the eye through the porus opticus. It immediately divides 



