418 THE SKIN AND THE SENSE-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, Schultze) 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 split 

 transversely and form small discs from 0-0005 0*00055 mm. thick ; 

 this takes place in the outer limb only. 



The inner segments of' the rods (Figs. 258, 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 Schultze) ; 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, Schwalbe). 



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

 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 mm. The inner segment is a long, thread-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 (macula lutea) 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 n), they are slightly 

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

 longitudinal striation (Sclmltze), 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 ju ; like the corresponding parts of the rods they 

 possess lens-shaped bodies at their junction with the outer segments, 



