838 



THE ORGANS OF SPECIAL SENSE. 



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 



f^gmentary layer. 



Jacob's membrane. 



- Membrana limitans externa. 



Outer nuclear layer. 



Outer molecular layer. 



Inner nuclear layer. 



Fibre of Mutter. 



Inner molecular layer. 



Vesicular layer. 

 Fibrous layer. 



Membrana limilans Interna. 

 FIG. 449. The layers of the retina (diagrammatic). (After Schultze.) 



wanting as a continuous layer ; (2) the ganglionic 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 pigmentary 

 layer, which is thicker and its pigment more pronounced than elsewhere. The 

 color of the macula 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 nervous layers of the retina terminate abruptly, and the 

 retina is continued onward as a single layer of elongated columnar cells covered by 

 the pigmentary layer. This prolongation is known as the pars ciliaris retina 1 , and 

 can be traced forward from the ciliary processes on to the back of the iris, where 

 it is termed the pars iridica retince or uvea. 



From the description given of the nervous elements of the retina it will be 

 seen that there is no direct continuity between the structures which form its 

 different layers except between the ganglionic and nerve-fibre layers, the majority 

 of the nerve-fibres being formed of the axons of the ganglionic cells. In the 

 inner molecular layer the dendrites of the ganglionic layer interlace with those 

 of the cells of the inner nuclear layer, while in the outer molecular layer a like 



