C38 



MANUAL OF HISTOLOGY. 



the substance of the internal members of the rods, close to their junction 

 with the outer halves (ellipsoids of Krause), are of not very infrequent 

 occurrence in apes (fig. 594, b), frogs, and water 

 newts (Schultze). The human cones possess them 

 also (Dobrowolsky). 



(2.) Having already devoted sufficient time to the 

 consideration of the membrana limitans externa in the 

 foregoing , we shall now turn to the next stratum. 



(3.) The external granular layer, stratum, granule- 

 sum externum consists of, besides the sustentacular 

 connective-tissue, several strata of small cells, whose 

 scanty body merely covers thinly their nuclei (fig. 

 595, A between a and d). The whole bed ranges in 

 thickness from 0*0501 to 0-0600 mm. over the greater 

 part of the retina, but decreases in depth both to- 

 wards the ora serrata and near the axis of the eye* 

 The cells of which this layer is made up are con- 

 nected both with the rods and cones, and have con- 

 sequently been classed into cone granules (fig. 589, b') 

 and rod granules (c'). The former are somewhat 

 pyriform or roundish oval in shape, and are remark- 

 able for their size (0-0090-0-0120 in length, 0-0041- 

 0-0061 mm. in breadth) : they also possess both a 

 large nucleus and nucleolus. They are never marked 

 by those dark transverse lines discovered recently by 

 Henle, which are only to be seen on the rod granules. 

 We have already described these bodies as applied 

 to the bases of the cones, so that nothing more need 

 be said here. The second species of granules are 

 smaller (0 '0045-0 '0079 mm.) oval, and generally 

 more numerous than the last. They are rarely applied 

 immediately to the bases of the rods, but are, as a 

 rule, connected with the latter by a shorter or longer 

 filament as the case may be. Those dark transverse 

 zones, already alluded to, which are observed on them (fig. 592, 1), are 

 still difficult to interpret. They are possibly post-mortem appearances. 

 They may, be single or multiple, but are usually double on each rod 

 granule (Henle, Hasse, Schultze). 



There now remain for our consideration the nervous fibre elements of 

 the inter-granular layer. 



As the fine fibres coming from the extremities of the rods enter the 

 corpuscles above, so do they leave the same at their opposite end, and 

 descending perpendicularly enter the inter-granular layer. Here they 

 appear to terminate with a fusiform or button-shaped swelling, exceeding 

 the ordinary varicosities in magnitude (fig. 595, B; fig. 589). But in 

 reality they have a longer course. Thus Hasse has in many cases seen a 

 delicate filament leaving this swelling, and losing itself in the inter- 

 granular layer. He looks, therefore, upon the little body as '' an inter- 

 polated ganglion cell." 



According to Schultze's observations, extremely delicate filaments may 

 be seen very distinctly to spring from these small swellings (at least in 

 birds and amphibia), and taking a horizontal course to become lost in the 

 tangle of the inter-granular layer. 



Fij?. 594. 0, human 

 cone with decomposed 

 external member, and 

 an apparently fibrous 

 internal half; 6, the 

 same from Macacus 

 ct/nomolgus, showing 

 division into plates of 

 the s'yle, and lenti- 

 cular structure in the 

 body. 



