Communication with other plerves.—The optic nerves have no direct communication with the other cerebral nervesAbut certain anatomists have traced filaments from the ganglionic system to them. Arnold (Icones nervorum Capitis, Tabula Sexta) has described and delineated two slender threads which run from the spheno-palatine or Meckel's ganglion to the optic nerve, and Hirzel observed in several instances the same arrangement. Tiedemann has seen an excessively delicate filament from the lenticular ganglion accompanying. the arteria centralis retinm through the optic nerve : he has also discovered branches of the ciliary nerves taking the same course, and ha.s even suc ceeded in following them as far as the retina; and M. Ribes (Memoires de la Societe Medi cale d'Emulation) has asserted, that a minute subdivision of the cavernous plexus extends along the arteria centralis retinm, being derived from that division of the plexus which accom panies the ophthalmic artery.
Organization.—The organization of the optic nerve is in many respects peculiar. Firstly. From the chiasma to its distal extremity it is enveloped by a strong coating of neurilemma, and from the inner surface of this tunic a number of processes are detached which divide the interior of the envelope into longitudinal canals wherein the medullary substance is lodged ; the optic nerve is not therefore a mere bundle of nervous cords (the structure prevalent in other nerves), but it is "a cylinder of collected tubes." Secondly. From the optic foramen to the sclerotic a sheath of dura mater is super added to the optic nerve, and since none of the other cerebral nerves possess a similar covering, it must be considered a special provision for the security of the second pair Thirdly. The arteria centralis retinm rims through the centre of the optic nerve (an anatomical arrangement of exceedingly rare occurrence): and the pri mitive fibres of the optic nerve evince a marked tendency to appear " varicose," a condition discovered by Ehrenberg, and considered by him and others peculiar to certain parts of the nervous system.
Real origin.—Anatomists have entertained very conflicting views upon this interesting question, so that from time to time different parts of the human encephalon have been considered the true origin of the optic nerves.
The older writers very generally believed that these nerves originate in the optic thalami, as the names " thalami nervorum opticorum " still applied to the bodies in question suffi ciently attest, and Eustachius, Varolius, Lieutaud, Haller, &c. supported this opinion.
Others conceived that the nates (or anterior pair of the tubercula quadrigemina) are the principal source of the optic nerves; this was maintained by Ridley, Winslow, Zinn, Morgagni, Sanctorini, Girardi, Hildebrandt, Boyer, Bichat, and Scemmerring ; and the same views were still more powerfully advocated by Gall and Spurzheim, although they admitted that the nerves derive a reinforcement from the corpora geniculata externa and the tuber cinereum.
Tiedemann (although fully aware that some filaments of the optic nerves are traceable to the surface of the optic thalamus both in the fcetus and adult) yet believed the mites and corpora geniculata externa to be tile true origins of the nerves under consideration, and in this opinion he was strengthened by the Report on the Memoir of Gall and Spurzheim, made to the Institute by Cuvier, Portal, Sabatier, and Pinel.
According to Serres the tubercula quadri gemina are the proper sources of the optic nerves, and by Leuret the second pair are traced to a triple cerebral attachment, viz. the nates, testes, and optic thalamus.
It is proposed to examine in this place some of the grounds on which the foregoing opinions have been founded, and to this inquiry the aid of comparative anatomy is indispensably re quisite.
Fisu.—In these animals the optic nerves are distinctly traceable to two of the ganglia which conipose the diminutive brain. The ganglia in question are called " optic lobes," from being the principal sources of the nerves of vision ; they are hollow, and their position in the brain is between the cerebral hemispheres and the cerebellum (fig.407.) The optic lobes in fish very generally bear proportion to the size of the optic nerves (a proof of their physiological relations); and this proportion becomes par ticularly apparent in fish which possess either unusually small organs of vision, as the Eel ; or eves of different dimensions, as the Pleuronectes.
In many kinds of fish the optic nerves de rive some of their- filaments limn a pair of tubercles placed on the under snrface of the encephalon beneath the optic lobes (fig. 410). The writer does not some to decide whether these tubercles are really identical with the millary eminences of the human brain as tained by Desmouhns and others; or with the tuber cinereurn, as Carus, Spurzheim, &c.' have' contended : but that they have a share I in the origin of the optic, nerves is certain, since in those fish which have two optic nerves of equal size, the tubercles1 to which allusion is made present correspond's% differences in dimen-, sions (fig. 409, g g).