CHAP. XVII.] THE OPTIC NERVES. 39 



nerve of the opposite side, decussating the similar fibres of the other 

 tract; and the outermost fibres, much fewer in number than the cen- 

 tral ones, pass to the optic nerve of the same side. 



This disposition of the fibres of the chiasma may be demonstrated 

 on a specimen which has been sufficiently hardened in spirit, by 

 tearing the fibres in their proper direction after the removal of the 

 neurilemma. By such a procedure, it may be shown that each optic 

 nerve derives its principal fibres from the tract of the opposite side, 

 and only a few fibres from those of its own side. 



The existence of such a decussation of fibres in the chiasma is, 

 moreover, rendered highly probable by the strong indications of 

 most extensive decussation, resembling that of the anterior pyra- 

 mids, in some of the large carnivorous birds, and also in the cross- 

 ing of the entire optic nerves in some of the osseous fishes, the cod 

 for example. 



In the common domestic quadrupeds the decussation of the fibres is 

 not to be made out so plainly, probably from its being more com- 

 plicated. The chiasma somewhat resembles a knotted union of the 

 two tracts, which is dense and firm in structure. 



The optic nerves appear also to be connected by fibres, forming 

 the anterior border of the chiasma, and which may be regarded as 

 commissural between the two retinae (a, fig. 126) . 



From the quadrigeminal tubercles to the chiasma, nerve-tubes, 

 mostly of large size, are visible by the microscope in the tracts. 

 In the chiasma and the optic nerves, the fibres, although very 

 variable in size, are so closely connected together that it is exceed- 

 ingly difficult to separate them. They seem F m 

 to be collected into numerous small bundles, 

 having an intricate plexiform interlacement 

 within the common sheath. Each bundle is 

 surrounded by a firm but dense neurilemma, 

 and thus it is impossible, by the ordinary 

 means of manipulation, to separate a portion 

 of the nerve of sufficient delicacy to examine 

 any considerable length of its fibres. The Fragments of Nerve Tubules 

 size and character of the fibres may be esti- ^^^S^S^ 

 mated by examining portions of them which SJSSHfiS^ttSS 

 project from the margin of the piece. 



The optic nerve is abundantly supplied with capillaries, which 

 form a network with elongated meshes in its substance. A little 

 behind the globe, it receives from the ophthalmic artery a branch, 

 before alluded to, the arteria centralis retinae, which penetrates to 



