398 FIFTH PAIR TRIFACIAL. 



cervical nerves, and is distributed to the trapezius. The spinal ac- 

 cessory sends numerous branches to the sterno-mastoid in its pas- 

 sage through that muscle ; its branches to the trapezius .may be 

 traced to the lower border of that muscle. 



The pneumogastric and spinal accessory nerves together resemble 

 a spinal nerve, the former representing the posterior root with its 

 ganglion, and the latter an anterior root. 



5th pair, TRIFACIAL (trigeminus). This nerve is analogous to the 

 spinal nerves in its origin by two roots, from the anterior and poste- 

 rior columns of the spinal cord, and in the existence of a ganglion 

 on the posterior root. Hence it ranges with the spinal nerves, and 

 is considered as the cranial spinal nerve. 



It arises* by two roots from a tract of yellowish white matter 

 situated in front of the floor of the fourth ventricle and the origin of 

 the auditory nerve, and behind the crus cerebelli. This tract divides 

 inferiorly into two fasciculi which may be traced downwards into 

 the spinal cord, one being continuous with the fibres of the anterior 

 column, the other with the posterior column. Proceeding from this 

 origin the two roots of the nerve pass forward, and issue from the 

 brain upon the anterior part of the crus cerebelli, where they are 

 separated by a slight interval. The anterior is much smaller than 

 the posterior, and the two together constitute the fifth nerve, which 

 in this situation consists of seventy to a hundred filaments held 

 together by pia mater. The nerve then passes through an oval 

 opening in the border of the tentorium, near to the extremity of the 

 petrous bone, and spreads out into a large semilunar ganglion the 

 Casserian. If the ganglion be turned over, it will- be. seen that the v 

 anterior root lies against its under surface without having any con- 

 nexion with it, and may be followed onwards to- the inferior maxil- 

 lary nerve. The Casserian ganglion divides into three branches, 

 the ophthalmic, superior maxillary, and. inferiqir maxiMar,y.. 



The OPHTHALMIC NERVE is a short trunk, being not more than " 

 three quarters of an inch in length ; it arises from the upper angle of 

 the Casserian ganglion, beneath the dura mater, and passes forwards 

 through the outer wall of the cavernous sinus, lying externally to 

 the other nerves: it divides into three branches. Previously to its 

 division it receives several filaments from the carotid plexus, and 

 gives off a small ' recurrent nerve, that passes backwards with the 

 recurrent branch of the fourth nerve between the two layers of the 

 tentorium to the lining membrane of the lateral sinus. 



The Branches of the ophthalmic nerve are, the 



Frontal, 



Lachrymal, 



Nasal. 



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

 result of his dissections, in the Cyclopoedia of Anatomy and Physiology. Mr. Mayo 

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







