664 



THE EYE 



face, the resulting retina would be composed of inverted elements (Fig. 

 562). Thus this theory at once offers an explanation for the two most 

 striking features of the vertebrate retina, namely, its formation as an 

 apparent outgrowth from the central nervous system and the inverted con- 

 dition of its receptive cells' (Parker). The difficulties of this theory are 



discussed, and an alternative 

 theory, based upon the direc- 

 tion eyes of Amphioxus, is 

 presented by G. H. Parker 

 (Amer. Nat., 42, 501, 1908). 



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 & 8 



THE OPTIC NERVE 



The optic nerve is a 

 large nerve trunk, com- 

 posed, like the white mat- 

 ter of the brain of which it 

 is an ontogenetic portion, 

 of medullated nerve fibers 

 without a neurolemma, sup- 

 ported by a neurogliar net- 

 work containing long-rayed 

 astroeytes. It receives an 

 investing sheath from each 

 of the cerebral membranes, 

 septa from the pia mater 

 enveloping the several funi- 

 culi. These sheaths are 

 continued as far forward 

 as the eyeball, at which 

 point they become contin- 

 uous with the sclera. 

 Though the choroid corre- 

 sponds to the pia mater, the 

 two are not apparently in 

 direct continuity. 



Lying in the axis of the 

 nerve, the arteria centralis 

 reiince with its accompany- 

 ing vein enters the eye and 

 appears on the inner sur- 



