70S 



mi: .v/://r/ -. 





With regard to its efWMalidll, tlic optic ncrvo enters tho globe of tlic 

 eye by piercing tho sclerotic and choroid coats, towards tho most de- 

 clivitous part of its posterior surface, and in the form of a membrane which 

 is described in ihe apparatus of vision as the retina. Before traversing tho 

 bottom of the eye, this nerve always becomes markedly constricted. 



Tho study of tho structure of tho optic nerve reveals some peculiar 

 facts which it is well to know, though they aro more curious than in- 

 teresting. The upper part is entirely destitute of envelope, while the 

 inferior that in front of the chiasma 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 presents 

 a multitude of septa (forming the lamina cribrosa), which keep tho fibres of 

 this nerve apart from each other. To make this organisation manifest, the 

 latter should be steeped in an alkaline solution for some days, and then wuslu-d 

 in 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 nervous tubules, and which 

 are formed by the internal 

 neurilemma, are then shown. 



Concerning the properties of 

 the optic nerve, we will say 

 nothing ; though they are analo- 

 gous to those of the other nerves 

 of special sense ; it is destined 

 to transmit to the encephalon 

 the impressions furnished by 

 the sense of sight, and mechani- 

 cal irritation of it docs not cause 

 pain. 



3. Third Pair, or Common Oculo- 

 motor Nerves. (Figs. 326, 335.) 



The nerves of tho third pair 

 emanato from the cerebral pe- 

 duncles, near the interpedtm- 

 cular fissure, and at an almost 

 equal distance between tho 

 corpus albicans and tho pons 

 NERVES OP THE EYE. Varolii. Their roots, seven or 



1, Ophthalmic branch of the fifth pair; 2, Palpebro- ei g ht in oaoh Penetrate the 

 nasal branch ; 3, Lachrymal nerve ; 3', Temporal texture of these peduncles, pass 

 branch of that nerve ; 4, Frontal nerve ; 5, Ex- backwards, and may be traced 

 ternal oculo-motor nerve; 6, Trochlear nerve; to their nucleus, which Stillim' 

 8, 9, 10, 11, Branches of the common oculo-motor i... _i___j u. 



nerve; '12,' Superior maxillary nerve; 13, Its J^S placed above tho anterior 

 orbital branches. border of the pons Varolu, and 



which is united to that of the 



opposite side by fibres intercrossing on tho median line. 



From the union of these roots results a flattened trunk, which is at first 



carried outward, and is almost immediately inflected forward to enter, along 



