MUSEUM OF COMPARATIVE ZOOLOGY. 15 



prising; for when we reflect upon the methods at his command, it is 

 remai'kable what success he had in tracing the course of the fibres, and 

 in demonstrating tiie relation of the cone-cells and rhabdome. 



Patten has advanced the view that the cone-cells are provided w'ith 

 an axial nerve-fibre, and that the cone itself is the true perceptive ele- 

 ment. I shall defer the consideration of this topic till I describe the 

 innervation of the retina. 



The Distal Hetinulce. 



Surrounding each crystalline cone are two pigment-cells, the distal 

 retinulse. These cells not only surround the cone, but extend as fibres 

 into the proximal part of tlie retina (Fig. 1, rt7i'. cht.). 



The relation which the distal retinulse sustain to the cone can be 

 studied most readily in transverse sections. In a section passing 

 through the distal end of the cone (Fig. 4, ommatidium a), it will be 

 observed that of the four lateral faces which the cone presents, two, the 

 lower and left-hand ones, are covered by a single retinula (rtn'. dst.). 

 The retinula is thickest at the lower left-hand angle of the cone, and 

 becomes thinner the farther it extends on the two adjacent faces. At 

 the more distant edges of these two faces the retinula terminates. 

 Thus the retinula is composed of a central portion and two blade-like 

 extensions. Each blade covers one face of the cone. The second 

 retinula is essentially like the one just described, but lies at the upper 

 right-hand angle of the cone and covers its upper and right-hand 

 faces. In this way the four faces of the cone are sheathed by a pair of 

 retinulcG. 



On inspecting the arrangement of the retinulse in adjoining omma- 

 tidia (Fig. 4), it is evident that they are so placed that the thick end 

 of each blade-like portion is opposite the thin end of the blade of a 

 neighboring retinula, and that, in passing along the space between the 

 cones, as one retinula becomes thicker the other becomes thinner. The 

 delicate membranes which separate the blades consequently extend 

 obliquely across the spaces between the cones (Fig. 4). In the space 

 w'hich is left between the angles of four adjoining cones the mem- 

 branes of the retinulse are very much thickened. That this is a 

 thickening in the membrane of the retinula, and not due to substance 

 produced by the cone-cells, seems probable for two reasons. First, the 

 membrane is often somewhat thickened in regions between two retinulce, 

 and where the cone-cells could not well touch it. Such thickenings are 

 directly continuous with the larger ones already mentioned (Fig. 4). 



