418 THE StvIN AND THE SEXSË-ORGANS. 



cause of this is not clearly understood ; some hold it to be a 

 fibre (Ritter^s fibres), others hold it to be an artificial product 

 (Hensen). 



After treatment with certain reagents the outer limbs show a 

 transverse striation, which is probably produced by the action of 

 these reagents on the sheath of the outer limbs ; that a sheath is 

 present is proved by its possession of a different refractive index 

 (Zenker, Sehultze) to the rest of the outer limb, and this transverse 

 striation is not seen until the whole organ has undergone consi- 

 derable post-mortem changes (Hoffmann). Should this change be 

 allowed to proceed a stage further, the outer limbs of the rods sj)lit 

 transversely and form small discs from o-ooo5-0"00055 mm. thick ; 

 this takes place in the outer limb only. 



The inner segments of the rods (Figs. 25H, 259) are short (0-020- 

 0-022 mm.) and of the same thickness as the outer limbs. When 

 perfectly fresh they appear homogeneous; very quickly changes 

 commence, which are probably due to coagulation. A plano-convex 

 figure (Fig. 259) is then seen at the outer portion of the seg- 

 ment (lens-shaped figure of Sehultze) ; with staining reagents it 

 gives the same reactions as the outer segment of the rods. The 

 rest of this segment forms a short cylinder, which probably has no 

 distinct sheath (Hoffmann, Merkel) ; some observers are inclined to 

 think that a sheath exists (Landolt, Sehwalbe). 



The outer segments of the rods are of two chief sizes (Sehwalbe). 

 Those of the one kind are large; the second variety occurs less fre- 

 quently, and the segments are shorter, measuring only 0*002 to 

 0-0025 '^ïïi- The inner segment is a long, thi-ead-like process, 

 except where it is swollen to enclose the lens-shaped body. 



The rods are much more numerous than the cones, except at one 

 small spot {imacula luted) on the posterior surface of the retina, 

 where only cones are found (Krause). 



The cones {coni) have each two segments like the rods (Figs. 258, 

 259). The outer segments are short (4-5 ju), they are slightly 

 conical and terminate externally in a blunt point ; they possess a 

 longitudinal striation (Sehultze), and very easily break up trans- 

 versely into small discs, which, however, do not separate so com- 

 pletely as in the case of the rods, in consequence of the presence of 

 a sheath continuous with a sheath on the inner segment. 



The inner segments (Figs. 258, 259) have convex sides and 

 measure 12-14 [i; like the corresponding parts of the rods they 

 jx)ssess lens-shaped bodies at their junction with the outer segments. 



