SECT. 226.] 



JACOB S MEMBRANE. 



553 



Fie. 22i. 



5, the limitary membrane ; these layers, with the exception of 

 the innermost lamina, which is everywhere equally thick, in 

 general become thinner as they are traced for- 

 wards, corresponding to the diminished thick- 

 ness of the retina as a whole. 



1. The layer of the rods and cones, stratum 

 bacillorum s. membrana Jacobi (fig. 224, 1), is 

 a very remarkable layer, composed extremely 



of innumerable strongly reflecting 



H 



fp 







. 





regularly 



bodies cvlindrical and conical in form. This 

 stratum in animals has hitherto (except by H. 

 Midler, vide infra) been quite incorrectly in- 

 terpreted, and even in man it has been but 

 very superficially known. It consists of two 

 elements, the rods, bacilli, and the cones, which 

 together form a single layer, whose thickness 

 at the bottom of the eye is o - 036", further on 

 diminishing to o'03o'", and quite anteriorly 

 becoming 0^028 " in thickness. These elements 

 are in general so arranged, that the more 

 numerous rods are directed with their thick 

 extremities outwards, while exactly the reverse 

 is the case with the cones; and it is on this 

 account that the latter, on imperfect investi- 

 gation, appear to constitute an inner, special, 

 narrower, layer situated between the inner ex- 

 tremities of the rods. The stratum bacillorum 

 terminates internally with a pretty sharp line, 

 the limitary line of the layer of rods, produced 

 by numerous lateral projections of its elements 

 abutting upon each other. 



The rods, (fig. 225, 2), in man, are long, nar- 

 row, cylindrical bodies, which possess the same breadth throughout 

 the whole thickness of the bacillar layer, and arc continued from the 

 inner end into the more internal layers of the retina, by means of 

 a thin process, or the filament of 2Iiillcr. Every rod is a cylinder, 

 o - 028'" to 0036'" long, 0"ooo8'" broad, transversely truncated at 

 the outer extremity, while the inner extremity tapers into a short 

 point, o'002" to 0003'" long, on the level of the limitary line 

 of the bacillar layer, the point being frequently marked off from 

 the rest of the rod by a delicate transverse line, and, therefore, 

 more properly belonging to the filament of the rod. From this 



Perpendicular sec- 

 tion of the human re- 

 tina, made six lines 

 anterior to the en- 

 trance of the optic 

 nerve; magnified 350 

 times. 1. bacillar lay- 

 er ; 2. outer granular 

 layer; 3. intermedi- 

 ate granular layer ; 

 4. inner granular lay- 

 er ; 5. finely granular 

 grey layer; 6. layer 

 of nerve - cells ; 7. 

 fibres of the optic 

 nerve ; 8. membrana 

 limitans. 





