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OPTIC NERVES. 



OPTIC NERVES. Under this beading it 

 is proposed to describe the special nerves of 

 vision in man, for although other names are 

 also employed by anatomists to denote the 

 nerves in question, the above is preferred as 

 being expressive of their functions. It should 

 be borne in mind that the optic nerves are like- 

 wise frequently called " the second pair ;" a 

 term derived from their numerical position on 

 the base of the brain, as they are the second 

 from before backwards on the under surface of 

 the er.cephalon. 



The anatomy and physiology of the optic 

 nerves in man constitute the more immediate 

 subjects of the present article, but as these 

 would be imperfectly treated without the aid 

 of comparative anatomy, the reader will find 

 in the following pages frequent references to 

 the condition of the nerves of vision in other 

 animals also. 



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 opticus. 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 crus, 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 chiasma, 

 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 arachnoid has a far less extensive 

 relation to the tractus opticus : in the early part 

 of its course the tractus has no serous covering, 



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 tractus 

 opticus is so closely connected to the crus 

 cerebri 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 with 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 tractus 

 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 infenorly, 

 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 tractus 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 cinereum, and to its 

 upper surface the peculiar greyish membrane 

 which closes up the third ventricle is adherent. 

 The chiasma has complex relations to blood- 

 vessels ; behind and below this body the ante- 

 rior portion of the coronary sinus is situated ; 

 external to the chiasma the termination of the 

 internal carotid artery is placed, and in front of 

 it are the anterior communicating, and a part 

 of the anterior arteries of the cerebrum. 



Optic nerve proper. This proceeds from 

 the chiasma, and after passing through the 

 foramen opticum into the orbit, and arriving at 

 the eye-ball, it perforates the sclerotic and 

 choroid coats, and terminates in the retina. 



First stage. In the short interval between 

 the chiasma and the optic foramen, the optic 

 nerve is directed forward and outward ; its size 

 is perceptibly greater than that of the tractus 

 opticus : it is not perfectly cylindrical in shape, 

 being slightly flattened above and below; it is 

 covered immediately by a dense tough neuri- 

 lemma, and provided besides with a distinct 

 sheath of arachnoid membrane, which, after 

 accompanying the nerve fairly into the hole in 

 the sphenoid bone, becomes reflected on the 

 process of dura mater lining that aperture. 

 Shortly after its commencement the optic nerve 

 is separated from the olfactory by the anterior 

 artery of the cerebrum. The ophthalmic artery 

 leaves the cranium by the foramen opticum also, 

 and lies beneath the optic nerve and to its outer 

 side, being there enveloped in a special sheath 

 of fibrous membrane. 



Second stage. Having entered the orbit, 

 the optic nerve inclines more directly forwards; 

 in consequence of this change of direction it 



