810 THE NERVES. 



These are the special nerves of smell. They receive the impression of odoure 

 and transmit it to the brain ; this function, which has been accorded and refused 

 them time after time, appears to be now definitively accepted. 



2. Second Pair, or Optic Nerves (Fig. 424). 



The nerves of vision present for consideration in their interesting study, their 

 origin, course, termination, and pro^yerties. 



There has been much dispute — and there will probably be much more — with 

 regard to the origin of the second pair. But without confining ourselves to an 

 appreciation of the opinions which have pervaded science on this matter, we will 

 describe what we have observed in the domesticated animals. 



When the isthmus is isolated from the brain (Fig. 426, 12) and examined 

 laterally, we recognize on its anterior limit the white band that constitutes the 

 optic nerve. Studied at its origin, this band is continuous, in the most evident 

 manner, with the external side of the thalamus 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 furnish- 

 ing the constituent fibres of the optic nerves.^ 



At first wide and thin, the optic band {tractus opticus) is rolled round the 

 cerebral peduncle downwards and forwards, and gradually narrows. Arrived at 

 the inferior surface of the brain, 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, 

 each passing 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 we have indicated. 



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

 peduncles and the hemispheres. From the point where they arrive free, at the 

 inferior face of the brain, to the commissure, they are covered by the pia mater, 

 and adhere by their deep face to the superior extremity of the peduncles. 



The commissure 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 greg root of the optic nerves. But 

 of all the proper connections of the commissure, 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 anastomosis ? 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 comniissm'e do not exclusively affect either of these arrange- 



' To the optic nerve has breii attributed two roots, which arc two portions of the small band 

 of that name. The external root arises in tiu; optic thalamus, tlie external corpus geniculatum, 

 and the anterior corpora quadrigemina; the internal leaves the internal corpus geniculatum. 

 According to certain authorities, the two roots pass to the game points; but others assert that 

 they reach the four corpora quadrigemina. 



