708 



TEE NUBVES. 



Fig. 335. 



With regard to its ermirmtion, the optic nerve enters the globe of the 

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

 clivitous part of its posterior surface, and 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 peculiar 

 facts which it is well to know, though they are 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 crihrosa'), which keep the 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 washed 

 in a stream of water to remove the softened nervous matter ; the nerve is 

 to be afterwards tied at one ot 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- 

 k cal irritation of it does not cause 

 pain. 



3. Third Pair, or Common Oculo- 

 motor Nerves. (Figs. 326, 335.) 



The nerves of the third pair 

 emanate from the cerebral pe- 

 duncles, near the interpedun- 

 cular fissure, and at an almost 

 equal distance between the 

 corpus albicans and the pons 

 Varolii. Their roots, seven or 

 eight in each, penetrate the 

 texture of these peduncles, pass 

 backwards, and may be traced 

 to their nucleus, which Stilling 

 has placed above the anterior 

 border of the pons Varolii, and 

 which is united to that of the 

 median line. 



flattened trunk, which is at first 

 inflected forward to enter, along 



ITEKVES OF THE EYE. 

 1, Ophthalmic branch of the fifth pair ; 2, Palpebro- 

 nasal branch ; 3, Lachrymal nerve ; 3', Temporal 

 branch of that nerve , 4, Frontal nerve , 5, Ex- 

 ternal oculo-motor nerve, 6, Trochlear nerve 

 8, 9, 10, 11, Branches of the common oculo-moter 

 nerve ; 12, Superior maxillary nerve ; 13, Its 

 orbital branches. 



opposite side by fibres intercrossing on the 



From the union of these roots results a 



carried outward, and is almost immediately 



