DESCRIPTIVE ANATOMY.
Apparent origin.—The optic nerves com mence by two broad medullary tracts (the tractus optici), each of which becomes first apparent at the under surface of the corres ponding, optic thalamus.
Tractus optieus.—This appears to derive its principal origin from the corpus geniculatum externum : from that tubercle a narrow band arises which is soon reinforced by another (not in general equally large or distinct,) from the corpus geniculatum internum, and by the junction of the two the tractus opticus is formed : thus constituted, the tractus opticus takes a course forward and inward around the outer and inferior surface of the crus cerebri : it is at first deeply concealed from view in the great cerebral fissure, being overlapped from without by the middle lobe of the cerebrum, so as to be invisible until a portion of the brain, together with the arachnoid membrane and pia mater, have been displaced. Emerging from under cover of the middle lobe, the tractus next gains the front of the erus, runs along the margin of the tuber cinereum, and at length unites with the other tractus opticus to form the chiasma.
At the crus cerebri the tractus opticus in creases in breadth, and of its two edges the anterior or external is here the thicker, while in the vicinity of the chiasma the tractus loses its flattened appearance, and becomes nearly cylin drical.
The tractus opticus is soft in texture through out, being devoid of the tough neurilemma from which the proper optic nerve derives its uncom mon firmness.
The tractus opticus receives a very extensive investment from the pia mater, which covers and adheres to all its free surface : anteriorly, where the tractus is approaching the chiasina, nearly two-thirds of its .circumference are clothed by pia mater; and further back, that membrane even insinuates itself a short distance between the posterior or inner margin of the tractus, and the adjacent surface of the crus cerebri. The amchnoid has a far less extensive relation to the tractus opticus : in the early part of its course the tractus has no serous covenng, but in the interval between the middle lobe of the brain and the chiasma, the arachnoid passes beneath the tractus opticus, and so affords it a partial investment.
The anterior or external margin of the trams opticus is so closely connected to the crus ccrebri that in attempts to separate them the medullary substance is torn, and consequently some anatomists are of opinion that the crus furnishes filaments of origin to the tractus; but the posterior or inner edge of the tractus is not identified svith the crus, for there the two structures can be separated without any violence to either.
The third and fourth nerves, before reaching the cavernous sinus, cross underneath the traetus opticus, but not immediately, for the serous and vascular membranes of the brain, and in general the edge of the middle cerebral lobe are interposed ; the posterior communicating artery passes also across the tractus inferiorly, and the artery of the choroid plexus, in its course to the great cerebral fissure, runs be neath it, the pia mater alone intervening between these bloodvessels and the tractus opticus.
The chiasma is somewhat quadrilateral, and receives by each posterior angle the cor responding tmctus opticus, while its anterior angles are prolonged respectively into either optic nerve; when" in situ," it is supported by a transverse groove of the sphenoid bone in front of the sella Turcica. Posteriorly, it is , identified with the tuber cinereuin, and to i upeer surface the peculiar greyish memb . which closes up the third ventricle is adher The chiasma has complex relations to blo vessels ; behind and below this body the an rior portion of the coronary sinus is situat external to the chiasma the termination of internal carotid artery is placed, and in front it are the anterior communicating, and a of the anterior arteries of the cerebrum.