THE RETINA. 265 



XII.— ON THK RETINA. 



The anatomical elements of the retina are most satisfactorily 

 examined in microscopic sections made at right angles to 

 the surface of the membrane, after maceration in dilute solu- 

 tion of chromic acid. "N'iewed in this manner, the retina 

 exhibits, from the peripheral to the central margin of a suc- 

 cessful section, a series of strata, which may be distinguished 

 as the bacillary, white cellidar, grey cellular, filamentary, and 

 limitaiy layers. 



The bacillary layer consists of two kinds of bodies — the 

 rods and cones. The rods are cylindrical or prismatic, with ex- 

 tremities transversely truncated, transparent, and of extremely 

 delicate texture. The cones only differ from the rods in 

 having their inner third, or two-thii-ds, pyriform. These are 

 arranged close together at right angles to the outer surface of 

 the retina, with their external extremities applied against the 

 iimer surface of the choroid. Throughout the greater pait of 

 the retina the rods predominate, the cones being unifoniih- 

 interspersed. In the neighbourhood of the yellow spot, the 

 cones become more freciuent; and in the spot itself, they 

 alone constitute the bacillary layer. 



The transversely truncated inner extremity of each of the 

 rods is connected with the deeper structures of the retina, 

 either by a conical appendage, which tapei-s inwards in the 

 form of a filament, or by an ovoidal appendage, which also 

 transmits a filament inwards. The inner extremity of each 

 nf llie cones is terminated by a pear-sluiped ai)pendage. 



