680 



AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



Conversely, the section of one optic tract causes a degeneration in both optic 

 nerves, the nerve of the side opposite to the lesion being most affected, and a 

 smaller degeneration appearing in the nerve of the same side (see Fig. 181). 



FIG. 181. Illustrating the relations of the afferent fibres in the optic nerve. The crossed fibres are 

 indicated by solid lines, the uncrossed fibres by broken lines : N, nasal side of the right eye ; T, temporal 

 side of the same ; Q. E, geniculatum externum : P, pulvinar ; C. Q, quadrigeminum anterius. 



It appears from this that in the higher mammals an optic tract is composed 

 of fibres from both optic nerves, but mainly of fibres from the nerve of the 

 opposite side. In the fish, amphibia, reptiles, and birds except the owls l 

 as well as in the lower mammals (mouse and guinea-pig, for example) the 

 decussation appears to be complete. 2 For the partial decussation in the owls 

 the evidence is physiological. This distribution of the optic fibres was asso- 



1 Ferrier : The Croonian Lectures on Cerebral Localization, London, 1890, p. 70. 



2 Singer und Miinzer : Dfnkschriften der malh.-naturwiss. Classe der kais. Akademie der Wissen- 

 schaften, 1888, Bd. Iv. 



