NERVES OF THE ENCEPHALON. 375 



der surface filaments, which, surrounding themselves with a 

 tunic from the dura mater, penetrate into the nose, and spread 

 themselves on the Schneiderian membrane. In its whole 

 length it is exceedingly soft and pulpy, ti|l it gets out of^the 

 cranium. 



The Optic Nerve or Second Pair (Nervus Opticus, Par Se- 

 cundum) is about the same size with the trigeminus. It arises 

 by a broad flattened root, one portion of which comes from the 

 posterior end of the thalamus opticus, and another from the 

 testis through the means of a medullary band that passes from 

 the latter towards the thalamus of the same side. From this 

 point the optic nerve winds forwards under the crus cerebri, 

 adhering to it and forming a connexion with the tuber cine- 

 reum, and then inclining inwards towards its fellow. Its ad- 

 hesion to the crus is considered by many anatomists as another 

 of its origins. 



The Optic Nerve, having reached the under anterior part 

 of the third ventricle, adheres so closely to its fellow that the 

 two seem fused together, in such a way that there is no line of 

 separation between them. This junction receives, above, from 

 the third ventricle, some medullary filaments, which Meckel 

 feels authorized to consider as another origin. The junction 

 presents the form of the letter X, and is called the chiasm or 

 crossing of the optic nerves. The most distinguished anato- 

 mists, however, have laboured in vain to settle the question of 

 the mode of junction; some believing that there was only a la- 

 teral union, others that the nerve of one side crossed over to 

 the other side, and others, again, that the decussation occurred 

 only with some of the fibres, but not all. Observations, in 

 comparative anatomy, on blindness, and indeed on every con- 

 ceived mode of elucidation, have been resorted to without 

 producing a solution of the problem; but the discussion of 

 their merits would require too much space for the present 

 work.* 



The Optic Nerves as they approach their chiasm become 

 more cylindrical, and, continuing so afterwards, penetrate into 



* Mr. Mayo, of London, considers the posterior part of the junction as formed 

 of fibres, making a curve, the convexity of which is forwards; it is said to be 

 very distinct in the mole, which has no other optic nerve. 



