438 PATHETIC. TRIFACIAL. 



along the external wall of the cavernous sinus ; and divides into two 

 branches which enter the orbit between the two heads of the external 

 rectus muscle. The superior branch ascends, and supplies the superior 

 rectus and levator palpebrae. The inferior sends a branch beneath the 

 optic nerve to the internal rectus, another to the inferior rectus, and a 

 long branch to the inferior oblique muscle. From the latter a short thick 

 branch is given off to the ciliary ganglion, forming its inferior root. 



The fibres of origin of this nerve may be traced into the grey sub- 

 stance of the crus cerebri,* into the motor tract, t and as far as the 

 superior fibres of the crus cerebri. J In the cavernous sinus it receives 

 one or two filaments from the cavernous plexus, and one from the 

 ophthalmic nerve. 



FOURTH PAIR. PATHETICI (trochlearifc). The fourth is the 

 smallest cerebral nerve ; it arises from the valve of Vieussens close to 

 the testis, and winding around the crus cerebri to the extremity of the 

 petrous portion of the temporal bone, pierces the dura mater near the 

 oval opening for the fifth nerve, and passes along the outer wall of the 

 cavernous sinus to the sphenoidal fissure. In its course through the 

 sinus it is situated at first below the motor oculi, but afterwards 

 ascends and becomes the highest of the nerves which enter the orbit 

 through the sphenoidal fissure. Upon entering the orbit the nerve 

 crosses the levator palpebrae muscle near its origin, and is distributed 

 upon the orbital surface of the superior oblique or trochlearis muscle ; 

 hence its synonym trochlearis. 



Branches. While nTftie cavernous sinus the fourth nerve gives off 

 a recurrent branch, some filaments of communication to the ophthalmic 

 nerve, and a branch to assist in forming the lachrymal nerve ; the 

 recurrent branch, which consists of sympathetic filaments derived from 

 the carotid plexus, passes backwards between the layers of the tento- 

 rium, and divides into two or three filaments, which are distributed to 

 the lining membrane of the lateral sinus. This nerve is sometimes a 

 branch of the ophthalmic, and occasionally proceeds directly from the 

 carotid plexus. 



FIFTH PAIR. TRIFACIAL (trigeminus). The fifth nerve, the great 

 sensitive nerve of the head and face, and the largest cranial nerve, is 

 analogous to the spinal nerves in its origin by two roots, from the 

 anterior and posterior columns of the spinal cord, and in the existence 

 of a ganglion on the posterior root. It arises from a tract of yel- 

 lowish white matter situated in front of the floor of the fourth ven- 

 tricle and the origin of the auditory nerve, and behind the crus 

 cerebelli. This tract divides inferiority into two fasciculi which may 

 be traced downwards into the spinal cord, one being continuous with 



* Mayo. f Solly. * Grainger. 



I have adopted the origin of this nerve, given by Dr. Alcock, of Dublin, as 

 the result of his dissections, in the Cyclopaedia of Anatomy and Physiology. 

 Mr. Mayo also traces the anterior root of the nerve to a similar origin. 



