THE FIELD OF VISION 35 



Rodents have no conjugate movements of the eyes, and 

 no consensual response of the pupils to light, only a direct 

 reaction. Singer and Miinzer* found that in mice and rats 

 the decussation of optic nerve fibers at the chiasma was 

 complete. Wilfred Harris examined the optic nerves, 

 chiasma and tracts of rabbits in sections stained with hema- 

 toxylin by Pal 's method, and also a fortnight after the experi- 

 mental enucleation of one eye, by staining with Marchi's 

 method, to trace the degenerated fibers. He found that 

 decussation was almost complete, only very few fibers, bere 

 and there, turning round from the optic nerve into the 

 optic tract of the same side. Ramon y Cajal 10 obtained 

 similar results. 



In the Ungulata it is difficult to observe the movements 

 of the two eyes at the same time, but it appears that they 

 are to some extent conjugate. In them the decussation of 

 the optic nerve fibers at the commissure is not complete. 

 In the horse and calf, two observers in Vienna, using Marchi's 

 method, found that about one-sixth of the fibers turn into 

 the optic tract of the same side. 



In the carnivorous mammals there is consensual reaction 

 of the pupils to light, and the movements of the two eyes 

 are more obviously conjugate than in the Ungulata. Wilfred 

 Harris found that experimental division of the optic tract 

 on one side in cats produced distinct homonymous hemi- 

 anopsia, with the hemianopic pupil reaction to direct light in 

 the opposite eye, though its consensual reaction was brisk 

 on exposure of the other eye to light. Microscopically he 

 found, in sections of the chiasma of a cat stained with 

 Marchi's method, after enucleation of one eye a fortnight 

 iously, that a large proportion of degenerate fibers were 

 I >i •< -cut in the optic tract of the same side. These degenerate 

 fibers were spread equally through all parts of the tract and 

 not collected into any distinct bundle. 



