812 



THE NERVES. 



3. Third Pair, or Common Oculo-motor Nerves (Figs. 424, 450). 



The nerves of the third pair emanate from the cerebral peduncles, near the 

 interpeduncular fissure, and at an almost equal distance between the corpus 

 albicans and the pons Varolii, in front of the loats niger. Their roots, seven 

 or eight in each, penetrate the texture of these peduncles, pass backwards, traverse 

 the red nuclei of Stilling, and may be traced to their nucleus, placed aV)ove the 

 anterior border of the pons Varolii. This nucleus is united, in the middle line, 

 to that of the opposite side. 



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 



with the sixth pair and the ophthalmic 

 Fig. 450. branch of the trigeminal nerve — the 



smallest of the great supra-sphenoidal 

 foramina — the sphenoidal fissure 

 {foramen lacerum basis cranii). The 

 common oculo-motov nerve afterwards 

 arrives, by the orbital hiatus, at the 

 bottom of the ocular sheath, where 

 it separates into several branches 

 destined to the following muscles of 

 the eye : the elevator of the upper 

 eyelid, superior rectus, internal rectus^ 

 inferior rectus, posterior rectus — ex- 

 cept its internal fasciculus — and the 

 small oblique. The branch to the 

 latter is remarkable for its great 

 length ; it reaches its destination in 

 passing to the outside of, and then 

 below, the inferior rectus. The 

 motor roots of the ophthalmic gang- 

 lion furnished by this nerve, are given 

 off from the same point as the branch 

 or the small oblique muscle. 



The nerves of the third pair are 

 purely motor, as is shown by their 

 connections with the inferior plane 

 of the cerebral peduncles, and their^ 

 exclusive distribution to contractile organs. They excite all the muscles lodged 

 in the ocular sheath, except the external rectus, the great oblique, and posterior 

 rectus. They also innervate the constrictive muscular fibres of the iris ; the dilating 

 fibres are supplied with nerves by the sympathetic. 



4. Fourth Pair, or Pathetici (or Trochlearis) (Fig. 450, 6). 

 The pathetic or internal oculo-motor (or trochlearis) nerve is the smallest of all 

 the cranial nerves. Its description is extremely simple. It arises from the band 

 of Reil, immediately behind the coi-pora quadrigemina, by two short roots, which 

 reach the anterior border of the valve of Vieussens, where are some cells, and from 

 there — their real nucleus, which is confounded with that of the conunon oculo- 

 motor — after intercrossing on the middle hue with the pathetic of the opposite 



KERVES OP THE EYE. 



I, Ophthalmic branch of the fifth pair ; 2, palpebro- 

 nasal br nch; 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-motor 

 nerve; 12, superior maxillary nerve; 13, its 

 orbital branches. 



