184 EECENT PROGKESS OF THE THEORY OF VISION. 



Bonders l proved that the same defects of accommodation are 

 the most frequent cause of squinting, and Von Graefe 2 had 

 already shown that neglected and progressive shortsightedness 

 tends to produce the most dangerous expansion and deformity of 

 the back of the globe of the eye. 



Thus connections were discovered, where least expected, 

 between the optical discovery and important diseases, and the 

 result was no less beneficial to the patient than interesting to 

 the physiologist. 



We must now speak of the curtain which receives the 

 optical image Avhen brought to a focus in the eye. This is the 

 retina, a thin membranous expansion of the optic nerve which 

 forms the innermost of the coats of the eye. The optic nerve 

 (Fig. 2, 0) is a cylindrical cord which contains a multitude of 

 minute fibres protected by a strong tendinous sheath. The 

 nerve enters the apple of the eye from behind, rather to the 

 inner (nasal) side of the middle of its posterior hemisphere. 

 Its fibres then spread out in all directions over the front of 

 the retina. They end by becoming connected, first, with 

 ganglion cells and nuclei, like those found in the brain ; and, 

 secondly, with structures not elsewhere found, called rods and 

 cones. The rods are slender cylinders; the cones, or bulbs, 

 somewhat thicker, flask-shaped structures. All are ranged per- 

 pendicular to the surface of the retina, closely packed together, 

 so as to form a regular mosaic layer behind it. Each rod is 

 connected with one of the minutest nerve fibres, each cone with 

 one somewhat thicker. This layer of rods and bulbs (also 

 known as membrana Jacobi) has been proved by direct experi- 

 ments to be the really sensitive layer of the retina, the structure 

 in which alone the action of light is capable of producing a 

 nervous excitation. 



There is in the retina a remarkable spot which is placed 

 near its centre, a little to the outer (temporal) side, and which 



1 Professor of Physiology in the University of Utrecht. 



2 This great ophthalmic surgeon died in Berlin at the early age of forty- 

 two. 



