740 



SPECIAL HISTOLOGY. 



Fig. SOS. 



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

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

 lowing layers from without 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. The bacillar layer, stratum bacillorum s. memb. Jacobi (Fig. 302 

 i, &), 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 (vid. 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 tnese 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, k, 2) are cylindrical, slender, 

 elongated corpuscles, in which 

 a larger external end, the pro- 

 per rod, is to be distinguished 

 from a more slender internal 

 portion, the prolongation 



or 



& u 



FiG. 303. Retinal elements of Man, magnified 350 diameters, 1, " rods" and radiating 

 fibres: &, proper " rod ;" r, prolongation of its pointed inner extremity; h, " granule'' (cell) 

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

 them to the surface of the optic layer ; //, " rod" seated on a t{ cone," t; r 7 , fibre proceeding 

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

 enlargement, Z, on the inner surface of the retina n, 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 : a, b, straight, 

 coarser and finer fibres, with varicosities ; c, without varicosities. 4, two " cones," 6, torn off 

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

 the " cones." 



