CHAP. XI.] PERIPHERAL NERVOUS END-ORGANS. 



457 



of the Visual 

 Epithelium. 



which travels through the more internal layers of the retina and 

 ultimately stimulates the optic nerve fibres. 



Particular The roc ^ s an( ^ cones are included by Klihne in the 



description term of visual cells (Sehzellen) or visual epithelium cells, 

 and must be distinguished from the retinal epithelium 

 cells, viz. the pigmented epithelium cells of the 10th 

 layer. Both rods and cones are distinctly nervous elements in so 

 far that they are doubtless in unbroken connection with the layer 

 from which nerve fibres ultimately spring. The rods and cones 

 possess some points in common and some which are distinguishing. 

 "Each consists of two distinct segments an inner and an outer; 

 the division between the two occurring, in the case of the rods, about 

 the middle of their length (in man); in the cones at the junction 

 of the finer tapering end-piece with the basal part ; consequently, the 

 outer and inner segments of the rods are nearly similar in size and 

 shape, the inner being, however, slightly bulged as a rule, whereas 

 the inner segment of the cone far exceeds the outer 

 one in size, the latter appearing merely as an appen- 

 dage of the inner segment (fig. 63). 



"The two segments both of the rods and cones ex- 

 hibit well-marked differences, both in their chemical 

 and optical characters, as well as in the structural ap- 

 pearances which may be observed in them. Thus while 

 in both the outer segment is doubly refracting in its 

 action upon light, the inner is, on the contrary, 

 singly refracting : the inner is stained by carmine, 

 iodine, and other colouring fluids, whilst the outer 

 remains colourless. The outer segment in both shews 

 a tendency to break up into a number of minute super- 

 imposed disks, whereas the inner segment is itself again 

 distinguishable into two parts an outer part, appa- 

 rently composed of fine fibrils, and an inner part, 

 homogeneous, or finely granular 1 ," by which they com- 

 municate directly, in the case of the rods with a rod- 

 fibre, in the case of the cones with a cone-granule. 

 The inner limbs of the rods are longer than those of 

 the cones ; on the other hand the outer limbs of the 

 cones are much shorter than those of the rods so that 

 the latter project above the former. 



Retinal Epi- The tenth layer (10, Fig. 62), or pig- 



theiium. mentary layer, is composed of a single 



layer of hexagonal pigmented epithelium cells, form- 

 ing a mosaic which covers the outer limbs of the 

 rods ; these cells are characterized by possessing long 

 processes which extend from their anterior faces in a 

 beard-like fashion, and lie in the crevices between the rods ^and cones. 



FIG. 63. A BOD 



AND CONE FROM 

 THE HUMAN RETI- 

 NA. (Schultze.) 



Quain, Elements of Anatomy, 8th ed. (edited by Schafer), Vol. n. p. 613. 



