36 EVOLUTION OF THE HUMAN EYE 



In monkeys and in man there are conjugate movements 

 of the eyes, and consensual reaction of the pupils to light, 

 as in the Carnivora, but in addition they have a very highly 

 developed spot of central vision with a special bundle of 

 nerve fibers leading from it, and highly developed powers 

 of convergence and accommodation. 



There is no very definite anatomical evidence as to the 

 course taken by the macular nerve fibers at the chiasma. 

 Cajal 10 discovered bifurcating fibers in the chiasma of the 

 rabbit, the divisions passing into the two optic tracts. 

 Kolliker 11 found similar bifurcations in the chiasma of a 

 cat a few days old. It was upon these observations that 

 Wilbrand based his hypothesis as to the escape of the macula 

 in cases of bilateral hemianopsia. 



He attributed it to the bifurcation of fibers coming from 

 each macula at the chiasma, so that both maculae were 

 connected with the occipital cortex on each side. There 

 is, however, no evidence as to where the dichotomous fibers 

 found in embryo cats come from, and in rabbits' retinre 

 no specialized central area, such as a macula, can be dif- 

 ferentiated. 



Usher and Dean 12 traced the course of degenerated nerve 

 fibers in the optic nerve and chiasma in monkeys after 

 experimental lesions in the retina at the macular region. 

 In summing up their results they say: "As regards the 

 question whether the macular fibers cross wholly or partially 

 in the chiasma, it is impossible from our experimental cases 

 to make a definite statement, owing to the difficulty that 

 exists in determining whether the wound was limited to 

 the macular fibers or, on the other hand, whether it had 

 damaged only a portion of these; while in Case 11 the decus- 

 sation was complete, in Case 10 there were uncrossed fibers, 

 even at the most posterior part of the chiasma." 



