274 MODE IN WHICH LIGHT ACTS ON THE RETINA. 



transverse sections, and their lengths. The continuous retinal 

 picture in the simple eye is psychically interpreted as a con- 

 tinuous image. If, therefore, the possessor of a compound eye 

 perceives a continuous image of an object, it must be the 

 result of a more complex psychical operation, in virtue of 

 which the separate portions of the ocular mosaic picture 

 are psychically combined, and interpreted as a continuous 

 whole. 



The successive researches of Treviranus, Gottsche, Han- 

 nover, Pacini, H. Muller, and Kolliker, have determined the 

 existence and general structure of close-set rods or columns, 

 which extend between the inner and outer surfaces of the 

 retina, in the midst of the nervous and vascular textures of 

 that membrane. The outer extremities of these rods present 

 a crystalline columnar aspect, and constitute, collectively, the 

 external layer of the retina, usually termed Jacob's membrane. 

 The ultimate filaments of the optic nerve, after being connected 

 in a plexiform arrangement in the ganglionic layer of the 

 retina, terminate each independently in the more perfect portion 

 of the retinal field, by passing into, or becoming continuous 

 with, the inner end or side of a rod. Kolliker considers these 

 rods as nervous structures that is, as terminal portions of the 

 nerve-filaments themselves ; and holds that they constitute the 

 parts of the nervous structure of the eye on which objective 

 light primarily acts. 



Having myself carefully examined the structure to which 

 I have now alluded, I have been able to verify the more im- 

 portant anatomical details, as described by their discoverers, 

 and agree with Kolliker in considering the rods as the primary 

 optic apparatus. I cannot, however, coincide with this dis- 

 tinguished observer in holding these rods as modified nerve- 

 filaments. I hold them to be special structures appended to 

 the extremities of the ultimate nerve-filaments, and referable 

 to the same category as the Pacinian bodies, touch-corpuscles, 



