208 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



The Histological Evidence. The histological results supple- 

 ment in a very satisfactory way the findings from physiology and 

 pathology. The retina itself, considered from an embryological 

 standpoint, is an outgrowth from the brain vesicles, and is there- 

 fore an outlying portion of the central nervous system. The optic 

 fibers, in terms of the neuron doctrine, must be considered as 

 axons of the nerve cells in the retina. If, therefore, an eye is enu- 

 cleated or an optic nerve is cut the fibers connected with the 

 brain undergo secondary degeneration and their course can be 

 traced microscopically to the brain. By this means it has been 

 shown that in man and the mammalia there is a partial decus- 

 sation of the optic fibers in the chiasma. The fibers from the 

 inner side of each retina cross at this point to the opposite optic 

 tract; those from the outer side of the retina do not decussate, 



Occipital lobe. 



Occipito-thalamic radiation. 

 Superior colliculus. 



Lateral geniculate. 

 Thalamus. 



Retina. 



Fig. 92. Diagram to indicate the general course of the fibers of the optic nerves and the 

 bilateral connection between cortex and retina. 



but pass into the optic tract of the same side. The fibers of the 

 optic tract end mainly in the gray matter of the lateral genicu- 

 late body, but some pass also to the thalamus (pulvinar) and 

 some to the superior colliculus of the corpora quadrigemina. 



