488 FIFTH PAIR OF NERVES. 



the countenance produced by the action of the muscle on which 

 they are spent. They arise froai the sides of the valve of the 

 brain, below and behind the tubercula Quadrigemina,* and are 

 so small that they appear like sewing threads. They proceed 

 round the crura of the cerebrum, and appear on the surface 

 between the pons Varolii and the middle lobes of the brain. 

 They proceed along the edge of the tentorium, which they per- 

 forate, and passing through the upper part of the cavernous 

 sinus, enter the orbit by the foramina lacera. They are exclu- 

 sively appropriated to the Superior Oblique or Trochlearis muscle. 



The Fifth Pair of Nerves, 



Are called Trigeminus, because each nerve divides into three 

 great branches. 



These nerves come out from the crura of the cerebellum 

 where they unite to the pons Varolii, by distinct fibres, which 

 are connected so as to form a cord or nerve, that is larger than 

 any other nerve of the brain. In many subjects this cord 

 seems partially divided into two portions, the anterior of which 

 is much smaller than the posterior, and appears softer at its origin. 



It passes into a short canal formed by the dura mater, near 

 the anterior extremity of the petrous portion of the temporal 

 bone, at a small distance below the edge of the tentorium. It 

 is perfectly loose and free from adhesion to the surface of this 

 canal : but it soon passes out of it under the dura mater, and 

 then adheres to that membrane. After leaving the canal, it 

 expands like a fan, but still consists of fine fibres, which have 

 some firmness. It is said that there are seventy or eighty of 

 these fibres in the expansion, but they appear to be more numer- 

 ous. Round the circumference of the expansion is a substance 

 of a brownish color, into which the fibres enter. This is the 

 Semilunar Ganglion, or the Ganglion of Gasser, and from it 

 the three nerves go off. 



— If the ganglion be carefully turned out of its bed it will be 

 found to belong only to the large posterior root of the nerve, 

 which is considered the sensitive tract. The small or anterior 

 root which is called the matartract, merely lies in contact with 



* See vol. ii. page 391. 



