740 



SPECIAL HISTOLOGY. 



retina, it is not more than 0*04 of a line in thickness, and finally ter- 

 minates quite ahruptly. Notwithstanding this various thickness, the fol- 

 lo^Ying layers from Avithout to within may be evidently distinguished in 

 all parts of it; 1, the layer of rods and cones [bacillar layer]; 2, the 

 granular layer ; 3, the layer of gray nerve-substance ; 4, the expansion 

 of the optic nerve ; and 5, the limitary membrane. These layers, with 

 the exception of the innermost, which is of uniform thickness through- 

 out, in general become thinner towards the front, in correspondence with 

 the diminished thickness of the whole retina. 



1. Tlie bacillar layer, stratum bacilloricm s. memh. Jacobi (Fig. 302 

 z, h), presents a very remarkable structure, being composed of innume- 

 rable rod-like and conical corpuscles, disposed with the utmost regularity 

 and reflecting the light very strongly. With the exception of H. Mul- 

 ler {yid. infra), this structure, in animals, has been understood quite 

 erroneously ; and even in Man it has been but very superficially known. 

 It consists of two elements — the rods, bacilli {k), and the cones, coni [i), 

 which together constitute a single layer, 0-036 of a line thick at the 

 bottom of the eye, more anteriorly 0*024, and quite in front not more 

 than 0-015 of a line in thickness. In general these bodies are so 



arranged that the more nume- 

 rous rods have their largest end 

 directed outwards, whilst the 

 cones are disposed in the re- 

 verse direction, whence the lat- 

 ter, when imperfectly examined, 

 appear to constitute an inner, 

 distinct, thinner layer, lying 

 between the inner extremities 

 of the "rods." 



In Man, the rods (Fig. 303, 

 1, /;;, 2) are cylindrical, slender, 

 elongated corpuscles, in wdiich 

 a larger external end, the pro- 

 pier rod, is to be distinguished 

 from a more slender internal 

 , portion, the prolongation or 



Fig. 303. — Retinal elements of Man, magnified 350 diameters, l,"rods"' and radiating 

 fibres: /t, proper "rod ;" r, prolongation of its pointed inner extremity; /i, " granule'' (cell) 

 of the outer granular layer; /, enlarged extremity of the radiating fibres proceeding from 

 them to the surface of the optic layer; A/, "rod" seated on a " cone," i; r', fibre proceeding 

 from the latter, connected with the " granule,"/, of the inner granular layer, and the terminal 

 enlargement, I, on the inner surface of the retina; w, one of the fibrous bundles in which the 

 radiating fibres frequently terminate at their innermost extremity. 2, "rods"' torn off from 

 their fibres, in various states of curvature, &c. 3, fibres of the optic nerve : «, fc, straight, 

 coarser and finer fibres, with varicosities ; c, without varicosities. 4, two " cones," b, torn off 

 from their processes, (f, with somewhat altered " rods," a, at their outer ends ; f, nucleus of 

 the " cones." 



