NERVOUS SYSTEM. 
65 
afterwards convicted of a second offence, and executed; and the 
nerve on the blind side was shrunk in every one of them. In 
birds, the optic nerves, which are exceedingly large, manifestly 
decussate \ and in fish, and particularly in those with a bony 
skeleton, they cross each other without mingling at all. 
Their Fibrous Structure.— The fibrous structure of the optic 
nerve is sufficiently evident in all our quadruped patients, and 
these fibres are so symmetrically arranged as to leave a canal in 
the centre of the nerve, which had been observed by some old 
anatomists, but the function of which had been misunderstood. 
It contains the central artery of the retina , derived from the 
ophthalmic artery, and which penetrates through the dura mater 
after the nerves leave the cranium, and enters with the optic nerve 
into the globe of the eye, for purposes which will be described 
when w 7 e treat of vision# This canal also contains the cential 
vein of the retina. 
The Reticulated Structure of the Optic Nerve —When, how¬ 
ever, I speak of symmetrical arrangement, do not misunderstand 
me/ I do not mean to say that the exact parallel direction of 
these fibres can always or scarcely ever be traced. It would 
seem to be necessary in such a nerve as this, of peculiar sensation, 
that there should be an intercommunication of nervous influence 
and matter through its whole extent, otherwise accident or disease 
might, oftener than it does, cause strange confusion in the im¬ 
pression conveyed ; therefore there is a continual exchange of 
minute branches going forward between the fibrils in every direc¬ 
tion ; in some nerves this reaches through their wdiole extent, in 
others it is more plainly to be traced in certain parts, and occasion¬ 
ally it is carried to such an extent, that the interior oi the nerve 
presents only a complicated reticular appearance. It is so heie : 
although we can trace the central canal plainly enough, the 
interchange of branches is so numerous, that the true parallel 
direction of the fibres cannot be distinguished. 
Termination oj the Optic Nerves. —Each nerve, after having 
separated from its fellow, pursues its course on its own side, 
escapes through the foramen opticum, and enters the cavity 
of the orbit: it there continues its path obliquely, penetrates 
between the muscles, and, particularly surrounded by the 
retractor muscle, reaches the inner and inferior and posterior part 
of the eyeball, the coats of which it penetrates, and, entering the 
globe of the eye, it expands over the choroid coat, and forms the 
retina. 
Sheathed in Dura Mater. —The dura mater usually quits the 
nerves as they emerge from the cranium, and they afterwards pur¬ 
sue their course surrounded and defended by their own neurilema: 
