THE COMPOUND ETES. 85 



Beneath the cornea are the cones (Plate VII., figs. 5, & 7) 

 These are clear conical transparent bodies, about l-700th of an 

 inch in length, which are surrounded by a membrane coloured 

 with bright red pigment, and containing nuclei. A clear basement 

 membrane exists between the substance of the cones and the 

 pigment membrane, and the latter is probably formed entirely 

 by the coalescence of pigment cells. The inner surface of the 

 lenses of the cornea and the bases of the cones correspond, each 

 lens having its own cone. 



The rods (Plate VII., figs 5 and 6), by which the cones are 

 continued to the nerve centre, are cylindrical, and connect the 

 apices of the cones with a layer of nerve cells beneath. They 

 are separated from each other by considerable spaces, traversed 

 by tracheal vessels, and filled with red pigment. Each rod is 

 nearly 1-1 00th of an inch in length. 



The rods consist of an external or cortical substance, which 

 forms the greater part of their thickness. A clear axis 

 cylinder, continuous with the axis of the cone, may be observed. 

 A third layer of transparent material appears to be inter- 

 posed between the axis cylinder and the cortical part of the 

 rods, and to extend as a thin layer into the cones between the pig- 

 mentary and transparent portions of those bodies. At the apex of 

 each cone and around the outer extremity of the corresponding 

 rod is a thickened ring of pigment, the use of which is unknown. 

 A nucleus-like body, refracting light strongly and coloured like 

 the pigment, is imbedded in the inner extremity of each rod. 

 It is best Been by hardening the part in a solution of chromic 

 acid. The inner extremities of the rods are imbedded in a 

 thin layer of nerve cells, which I shall call the common retina. 



Little can be said concerning the uses of the parts above 

 described. I think it not improbable, that the hollow cylinders 



