376 NERVOUS SYSTEM. 



the orbits through the foramina optica. It is only in front of 

 their junction that they are invested by a neurileme; which, 

 having considerable firmness, penetrates into their interior, and 

 divides them into distinct canals. 



The Nervus Motor Oculi, or Third Pair (Par Tertium,) 

 arises from the internal face of the crus cerebri, about two lines 

 in advance of the anterior margin of the tuber annulare. Its 

 roots come, in great part, from the cineritious matter which is 

 found on the surface of the crus, and may be traced for some 

 distance upwards and backwards along the parietes of the third 

 ventricle. The nerves of the opposite sides are in contact for 

 some distance by the internal faces of their roots, but do not 

 adhere.* 



The Nervus Motor Oculi proceeds from its origin towards- 

 the external margin of the cavernous sinus, and, penetrating into 

 the orbit through the sphenoidal foramen, it is distributed to most 

 of the muscles of the eye-ball. 



The Pathetic Nerve, or Fourth Pair (Nervus Patheticus, Par 

 Cerebrate Quartum,) is the smallest which comes from the en^ 

 cephalon, and is not larger than a sewing thread. It arises by 

 two filaments, or roots, from the .upper anterior face of the 

 valve of the brain, just below the testes. This origin is soft, 

 and easily broken, from the want of a neurileme;. but the latter 

 is soon afterwards furnished. 



The Nervus Patheticus appears on the base of the brain, be- 

 tween the cerebellum and the posterior lobes of the cerebrum, 

 at the external margin of the tuber annulare. It then goes for 

 some distance along the margin of the tentorium till it comes 

 near the posterior clinoid process : it then penetrates into a ca- 

 nal of the dura mater, and reaches the orbit of the eye through 

 the sphenoidal foramen,, to be distributed on the superior oblique 

 muscle. 



The Nervus Trigeminus, or Fifth pair, also called Trifacial, 



* Mr. Solly considers this nerve to arise from the interior of the pons, and to 

 be connected with the valve of Vieussens by some fibres, from the latter going 

 into the crus cerebri. 



