OPTIC NERVE. THIRD PAIR. 



593 



section it is seen to consist of a number of separate bundles of nerve fibres, 

 imbedded in tough fibrous tissue prolonged from the dura mater, and per- 

 forated in the centre by the small arteria centralis retinae, which passes into 

 it soon after it enters the orbit. It is surrounded by the recti muscles, 

 and, entering the eyeball posteriorly a little to the inside of its middle, it 

 pierces the sclerotic and choroid coats, and expands in the retina. (See the 

 Anatomy of the Eye.) 



It may be mentioned that in many fishes the optic nerves do not unite in a com- 

 missure, but merely cross each to the side opposite to that of its origin ; and that 

 in a number of the same animals, as was first pointed out by Malphighi, the nerve 

 consists of a lamina thrown into complicated longitudinal plications, and surrounded 

 by a sheath. 



THIRD PAIR OF NERVES. 



This nerve, the common motor nerve of the eyeball (motorius oculi), 

 gives branches to five of the seven muscles of the orbit, viz., to the 



Fig. 402. VIEW FROM ABOVE Fig. 402. 



OF THE UPPERMOST NERVES 

 OF THE ORBIT, THE GAS- 

 SERIAN GANGLION, &c. (from 

 Sappey after Hirschfeld and 

 Leveille). | 



I, the olfactory tract passing 

 forwards into the bulb ; II, the 

 commissure of the optic nerves ; 

 IN, the oculo-motor ; IV, the 

 trochlear nerve; V, the greater 

 root of the fifth nerve, a small 

 portion of the lesser root is 

 seen below it ; VI, the sixth 

 nerve ; VII, facial ; VIII, audi- 

 tory ; IX, glosso-pharyngeal ; 

 X, pneumo-gastric ; XI, spinal 

 accessory ; XII, hypoglossal ; 

 1, the Gasserian ganglion; 2, 

 ophthalmic nerve ; 3, lachrymal 

 branch ; 4, frontal ; 5, external 

 frontal or supraorbital ; 6, 

 internal frontal ; 7, supra- 

 trochlear branch ; 8, nasal 

 nerve ; 9, infratrochlear branch ; 

 ] 0, internal nasal passing 

 through the internal orbital 

 foramen; 11, anterior deep 

 temporal proceeding from the 

 buccal nerve ; 12, middle deep 

 temporal ; 13, posterior deep 

 temporal arising from the 

 masseteric ; 14, origin of the 

 temporo-auricular ; 15, great superficial petrosal nerve. 



superior, internal and inferior straight muscles, to the levator palpebrse, and 

 to the inferior oblique muscle. 



Cylindrical and firm, like the other motor nerves, the third nerve, 

 quitting the investment of the arachnoid membrane, pierces the inner layer 

 of the dura mater close to the posterior clinoid process, and proceeds 

 towards the sphenoidal fissure, lying in the external fibrous boundary of the 

 cavernous sinus. 



