254 TWENTY-FO UR TH LECTURE. 



Lateral branches of the latter lead to manifold communica- 

 tions. In the molecular (gf) and the intergranular layer (d) 

 there is thus formed a very fine reticular frame-work, such as 

 we are already familiar with in the gray substance of the cere- 

 brospinal system (p. 220). 



Nuclei or cell equivalents occur occasionally in the system 

 of supporting fibres, as in the external granular layer (<?'). 



The supporting substance certainly extends as far as the 

 base of the rod and cone layer (a). There is scarcely 

 any doubt, however, that it extends still further as a delicate, 

 homogeneous connecting substance. At the former locality 

 it forms, as the membrana limitans externa, a fenestrated 

 boundary layer, further outwards a connecting medium of 

 the rods and cones. 



Having thus become familiar with the connective-tissue sub- 

 stratum — it should by no means be genetically confounded 

 with the ordinary connective tissue — let us pass to the ner- 

 vous elements of the retina (B). Let us here select the re- 

 versed course, and commence with the outer layer. 



This stratum is formed by the rods and cones. The whole 

 layer is called the rod-layer, stratum bacillosum. They are 

 terminal nerve cells, similar to those which we previously met 

 with at the higher nerves of sense. Those of the retina, 

 however, possess many peculiarities, and we have a more ac- 

 curate knowledge of them than of their relatives. The cir- 

 cumstance is also interesting that the rods and cones vary 

 according to animal groups. Their dimensions are propor- 

 tionate to that of the red blood cells. 



The rods, bacilli (B, b), are slender cylindrical structures. 

 They consist (Mueller, Braun, Krause) regularly of two parts, 

 an apparently homogeneous narrower so : called " outer mem- 

 ber," which refracts the light more strongly, and a shorter 

 " inner member." The latter appears paler, somewhat granu- 

 lar, and of considerable diameter. In the lower vertebrate 

 animals the retinal pigment forms regular sheaths around the 

 outer member of the rods and cones. In mammals and man 

 the pigment sheath is less developed. 



