THE EYE. 



255 



The rods acquire their greatest length, 0.06 mm. and more, 

 at the bottom of the retina. Further forwards they become 

 shorter, towards the ora serrata they are only 0.0399 mm. 

 high. Their diameter may be estimated at 0.0016 to 

 0.0018 mm. 



Downwards or inwards, beneath the membrana limitans 

 externa, the rod becomes pointed, and runs out into an ex- 

 traordinarily fine filament, a primitive 

 nerve fibrilla (Fig. 203, B, Fig. 204, 1, 

 4, Fig. 205, 1, .3). The latter passes 

 through the outer granular layer verti- 

 cally (and also radially). A small cell, 

 the so-called " rod granule" (Fig. 203, 

 B, b\ Fig. 204, 1, 2, 3, Fig. 205, 3) is 

 embedded in its course, sometimes 

 higher up, sometimes further down- 

 wards. This granule forms the single 

 element of the outer granular layer. 



Still more complicated textural con- 

 ditions have been observed in the rods 

 (Fig. 204). At the border of the in- 

 ner member towards the outer mem- 

 ber, embedded in the former, a plano- 

 convex body has been found with its 

 plane base directed upwards (1, a, 2). 

 " rod-ellipsoid " of Krause. 



Furthermore, as has been long known, the outer member 

 breaks up into transverse plates (3). These discs may have 

 a thickness of 0.0003 to 0.0004 mm. in man (Schultze). 



The outer member shows a longitudinal striation, caused 

 by longitudinal, channel-like depressions, with longitudinal 

 elevations springing up between them, like a fluted column 

 (Fig. 204, i, 2, and Fig. 205, I a). Longitudinal striations 

 have also been subsequently discovered on the inner members 

 (Fig. 205, 1, and 3 b). In the axis of the rod a very fine 

 filament, a primitive nerve fibrilla, is also said to have been 

 noticed (Ritter). 



Fig. 204. — Final structure of 

 the rods ; I, from the chicken 

 with the outer and inner mem- 

 ber, as well as the cone-ellip- 

 soid ; 2, from the frog ; 3, the 

 outer member of the rod of a 

 frog dividing into transverse 

 discs ; 4. rod with granule 

 from the Guinea-pig. 



This is the so-called 



