NERVOUS CONSTITUENTS OF THE RETINA. 267 



my measurement, have almost the same thickness as in Vertebrata, i.e. 

 about 0'5 of a microinillimeter. 



2. In the intervening spaces between these palisades, and upon 

 their surface, are fine fibrils, the extremities of the optic fibres. These, 

 proceeding from a layer of nucleated fusiform bodies comparable 

 to an external granule layer, enter the bacillar layer, whilst the 

 nucleated fusiform bodies that terminate by one of their extremities 

 in the fibres of the bacillar layer break up at their other extremity 

 into fibrils which proceed from the optic layer. 



3. In this layer the granular brownish-black pigment is also to be 

 included. This is never absent at the outer extremities of the rods, 

 and here separates these last from the fusiform bodies. This pig- 

 ment, like the nerve fibres, lies external to the lamellated palisades 

 which it invests, then extends further in the intervening spaces occu- 

 pied by nerve fibres between the palisades, and frequently forms at 

 the inner ends of the rods, where these are separated from the vitreous 

 by a homogenous membrane, a dense accumulation filling up the spaces 

 between the palisades. Light can penetrate into the latter, but is 

 excluded by the thick layer of pigment from the canals containing 

 nerve fibrils, or reaches these only by a circuitous path through the 

 lamellar palisades." 



It is evident that if the fine fibrils running in Vertebrata upon the 

 surface of the outer segments of the rods and cones are nerve fibrils, 

 their position on the one hand in relation to the lamellated substance, 

 and on the other to the pigment of the pigmented epithelial cells of 

 the retina, would be precisely identical] with that of the analogous 

 structures of the retina of the Cephalopoda. 



In the eyes of Articulata the structure of the retina is complicated 

 in accordance with the circumstances of these organs, being here 

 composed of many single eyesf united into one ; but here also 

 lamellated rods J are found behind the refractile bodies comparable 

 with the cornea, lens, and vitreous, with extraordinary powers of 

 reflexion, and very often of considerable length. They also are 

 invested by dark pigment, and stand in close relation to nerve 

 fibrils, which enter at their posterior extremity, and terminate either 



* See especially Max Schultze, Archiv fur Mikroskop. Anatomie, Band 



v., pp. 15 18. 



f Leydig, Das Auge der Gliederthiere, " The Eyes of Articulata." 



| Max Schultze, Untermchungen ueber die zusammengesetzten Augen 



dcr Krebse und Insecten, " Researches on the compound eyes of Crabs 



and Insects." Bonn, 1868. 



