226 



THE NERVOUS SYSTEM 



while the axons form the innermost stratum of the retina, the stratum opticum, 

 through which they enter the optic nerve. It will be apparent from Fig. 160 

 that the visual cells are the receptors and neurons of the first order in the optic 

 path. The impulses are transmitted through the bipolar cells to the ganglion 

 cells, whose axons, in turn, carry them by way of the optic nerves to the supe- 

 rior colliculus, lateral geniculate body, and pulvinar of the thalmus. In the same 

 figure it may be seen that the nerve also contains some efferent fibers which 

 terminate in the retina (Arey, 1916). 



The Optic Chiasma and Optic Tracts. The optic nerve emerges from the 

 bulbus oculi at the nasal side of the posterior pole and, after entering the cranium 

 through the optic foramen, unites with its fellow of the opposite side to form the 



Pulvinar of thalamus Aqueduct of cerebrum Red nucleus 

 Medial geniculate body, 



' \ .^fr ' *'' *j5^L/* > \^ ubstantia ni & ra 



Lateral geniculate body ^F ^Mlr ""* ''' iBPL X^lfc '' 'Base of peduncle 



\ 







Cerebral peduncle '' 



Optic tract -'xj 



4Q 

 Posterior perforated substance '' 



Mammillary body 



* Anterior perforated substance 



Tuber cinereum 



Optic nerve 



'Olfactory trigone 



\ 



| Ynfundibulum 



Optic chiasma 



Fig. 161. The connections and relations of the optic tracts. The mesencephalon has been cut 

 across and the specimen is viewed from below. (Sobotta-McMurrich). 



optic chiasma, in which a partial decussation of the fibers takes place (Figs. 

 161, 162). Beyond the decussation fibers from both retinae are continued in 

 each of the optic tracts. In the chiasma the fibers from the two optic nerves 

 are so distributed that each tract receives the fibers from the lateral half of the 

 retina of its own side and those from the medial half of the opposite retina. 

 The optic tracts partially encircle the ends of the cerebral peduncles. Each 

 tract divides into a medial and a lateral root, of which the former goes to the 

 medial geniculate body and does not consist of optic nerve-fibers. The lateral 

 root is much larger and runs to the lateral geniculate body and pulvinar of the 

 thalamus and to the superior colliculus of the corpora quadrigemina. In addi- 

 tion to the optic fibers each tract contains a bundle of fibers, known as the com- 



