THE CRANIAL OR ENCEPHALIC NERVES. 811 



ments ; for in studying them iu a specimen that has been macerated for 8ome 

 days, it is found that the majority cross each other in a very evident manner, 

 but that a part regain the ner\e corresponding to the side from which they came. 

 It is therefore seen that the nerves of the second pair are composed of one kind 

 of fibres on this side of the commissure, while beyond it they show two kinds — 

 the fibres from the right and left sides. The majority, we have said, cros^ 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, when an eye is lost from the ravages of 

 periodic ophthalmia {Jfuxioii jieriodique), the consecutive atrophy of the optic 

 nerve nearly always stops at the commissure, though it sometimes happens that 

 it gets beyond this ; and it is observed that it is usually the nerve opposite to 

 the diseased eye which suffers the most. Otherwise, the arrangement just de- 

 scribed is only a degree less advanced than that remarked in certain species — in 

 the osseous fishes, for instance — in which the optic nerves entirely cross each 

 other without confounding or mixing their fibres. 



But behind the ganglionic centre of these nerves, contained in the corpora 

 geniculatum and quadrigemiua, the direct fibres of the optic nerves cross each 

 other, and reach — along with those that crossed at the commissure — the sensitive 

 centre spread in the grey substance of the posterior lobe of the brain. This is 

 the opinion of Gudden, Nicati, Charcot, Landolt, etc. ; they compare the optic 

 nerves to the other cranial nerves. 



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 

 oruli), which envelops each nerve as in a sheath. In the orbital cavities they 

 are also related to some other nerves and vessels. 



With regard to its termination, the optic nerve enters the globe of the eye 

 by piercing the sclerotic and choroid coats, towards the most declivitous part of 

 its posterior surface, and expands in the form of a membrane, which is described 

 in the apparatus of vision as the retina. Before traversing the bottom of the 

 eye, this nerve always becomes markedly constricted. 



The study of the structure of the optic nerve reveals some pecuUar facts 

 which it is well to know, though they are more curious than interesting. The 

 upper part is entirely destitute of envelope, while the inferior — that in front of 

 the commissure — has a double neurilemma. The external layer of this is only 

 a dependency of the dura mater— a kind of fibrous sheath attached at one end 

 to the margin of the optic foramen, and at the other to the sclerotica. The 

 internal, which is analogous to the neurilemma of the other nerves, emanates 

 from the pia mater, and shows a multitude of septa (forming the lamina cribrosa), 

 which keep the fibres of this nerve apart from each other. To make this 

 organization manifest, the nerve should be steeped in an alkaline solution for 

 some days, and then washed iu a stream of water to remove the softened nervous 

 matter ; the nerve is to be afterwards tied at one of its extremities, inflated, tied 

 at the opposite end, and dried. By means of some sections, all the canals that 

 lodge the fasciculi of nerve-tubules, and which are formed by the internal 

 neurilemma, are visible. 



Concerning the properties of the optic nerve, we will say nothing ; though 

 they are analogous to those of the other nerves of special sense. It transmits to 

 tlie brain the impressions furnished by the sense of vision, and mechanical irrita- 

 tion of it does not cause pain. 



