THE CEANIAL OS ENCEPBALIC NERVES. 707 



opticus, where it forms the two enlargements known as the corpora geniculata. 

 This thalamus ought, therefore, to be regarded as the point of departure of 

 the nerve that bears its name. But as the external corpus geniculatum is 

 in contact with the natis, and as the internal is united to the testis by a 

 band of white fibres, it is almost certain, according to several authorities, that 

 the corpora quadrigemina concur in furnishing the constituent fibres of 

 the optic nerves. 



At first wide and thin, the optic band (trachis opticus) is rolled round 

 the cerebral peduncle from above to below and behind to before, and 

 gradually narrows. Arrived at the inferior surface of the encephalon, it 

 is changed into a funicular cord, which unites with that of the opposite 

 side to form the commissure or chiasma of the optic nerves ; this is, only a 

 temporary fusion, as beyond it the two nerves reappear, and pass into the 

 optic foramen, to reach the interior of the ocular sheath and the bottom 

 of the globe of the eye. 



We will enter into some details on the relations of the optic nerves 

 in the different points of the course .ve have indicated. 



In their flat portion, or origin, they are comprised between the cerebral 



peduncles and the hemispheres. From the point where they become free, 



at the inferior face of the encephalon, to the chiasma, they are covered by 



the pia mater, and adhere by their deep face to the superior extremity of 



■ the peduncles. 



The chiasma is lodged in the optic fossa, and receives on its deep face 

 the insertion of the small grey lamina which bounds the third ventricle in 

 front ; for which reason this is generally described as the grey root of the 

 optic nerves. But of all the proper connections of the chiasma, the most 

 important are certainly those which each nerve maintains with its congener 

 at their junction. What becomes of the fibres of each nerve in this anas- 

 tomosis ? Do they cross one another to reach the opposite eye ; or do they 

 merely lie together, and afterwards separate, in order to go to the eye on 

 their own side ? Anatomy demonstrates that the fibres of the chiasma do 

 not exclusively affect either of these arrangements ; for in studying them 

 in a specimen that has been macerated for some days, it is found that the 

 majority cross each other in a very evident manner, but that a part regain 

 the nerve corresponding to the side from which they came. It is there- 

 fore seen that the nerves of the second pair are composed of one kind of 

 fibres on this side of the chiasma, while beyond it they show two sorts — 

 the fibres from the right and left sides. The majority, we have said, cross 

 each other ; and the proof of this is afforded in certain facts observed in 

 pathological anatomy, which are of sufficient interest to be mentioned 

 here. In the cases so frequently occurring in the Horse, where an eye is 

 lost from the ravages of specific ophthalmia (fluxion periodique), the 

 consecutive atrophy of the optic nerve nearly always stops at the chiasma. 

 though it sometimes happens that it gets beyond this ; and it is observea 

 that it is usually the nerve opposite to the diseased eye which suffers the 

 most. Otherwise, the arrangement just described is only a degree less 

 advanced than that remarked in certain species— in the osseous flshes for 

 instance— in which the optic nerves entirely cross each other without con- 

 founding or mixing their fibres. _ . , . 



Beyond their commissure, the nerves of the second pair are in relation 

 with the walls of the optic foramina, then with the posterior rectus muscle 

 (retractor oculi), which envelops each nerve as in a sheath. In the orbits 

 they are also related to some other nerves and vessels. 



