THE FIFTH OR TRIFACIAL NERVE. 725 



Emerging from the upper end of the valve of Vieussens, the nerve is directed 

 outward across the superior peduncle of the cerebellum, and then winds forward 

 round the outer side of the crus cerebri, immediately above the pons Varolii, 

 pierces the dura mater in the free border of the tentorium cerebelli, just behind, 

 and external to, the posterior clinoid process, and passes forward in the outer 

 wall of the cavernous sinus, between the third nerve and the ophthalmic division 

 of the fifth. It crosses the third nerve and enters the orbit through the sphe- 

 noidal fissure. It now becomes the highest of all the nerves, lying at the inner 

 extremity of the fissure internal to the frontal nerve. In the orbit it passes 

 inward, above the origin of the Levator palpebrge, and finally enters the orbital 

 surface of the Superior oblique muscle. In the outer wall of the cavernous 

 sinus this nerve is not infrequently blended with the ophthalmic division of the 

 fifth. 



Branches of Communication. In the outer wall of the cavernous sinus it 

 receives some filaments from the cavernous plexus of the sympathetic. In the 

 sphenoidal fissure it occasionally gives off a branch to assist in the formation of 

 the lachrymal nerve. 



Branches of Distribution. It gives off a recurrent branch, which passes back- 

 ward between the layers of the tentorium, dividing into two or three filaments 

 which may be traced as far back as the wall of the lateral sinus. 



Surgical Anatomy. The fourth nerve when paralyzed causes loss of function in the 

 Superior oblique, so that the patient is unable to turn his eye downward and outward. Should 

 the patient attempt to do this, the eye on the affected side is twisted inward, producing diplopia 

 or double vision. Accordingly, it is said that the first symptom of this disease which presents 

 itself is giddiness when going down hill or in descending stairs, owing to the double vision 

 induced by the patient looking at his steps while descending. 



The Fifth Nerve. 



The Fifth or Trifacial Nerve (n. trigeminus) is the largest cranial nerve. It 

 resembles a spinal nerve (1) in arising by two roots ; (2) in having a ganglion 

 developed on its posterior root ; and (3) in its function, since it is a compound 

 nerve. It is the great sensory nerve 01 the head and face and the motor nerve 

 of the muscles of mastication. Its upper two divisions are entirely sensory ; the 

 third division is partly sensory and partly motor. It arises by two roots: of these 

 the anterior is the smaller, and is the motor root ; the posterior, the larger and 

 sensory. Its superficial origin is from the side of the pons Varolii, nearer to the 

 upper than the lower border. The smaller root consists of three or four bundles ; 

 the larger root consists of numerous bundles of fibres, varying in number from 

 seventy to a hundred. The two roots are separated from one another by a few of 

 the transverse fibres of the pons. The deep origin of the larger or sensory root 

 is chiefly from a long tract in the medulla, the lower sensory nucleus, which is 

 continuous below with the substantia gelatiuosa of Rolando. The fibres from this 

 nucleus form the so-called ascending root of the fifth ; they pass upward through 

 the pons and join with fibres from the locus cseruleus or upper sensory nucleus, 

 which is situated to the outer side of the nucleus, from which the lower part of 

 the motor root takes origin. The deep origin of the smaller or motor root is 

 derived partly from a nucleus embedded in the gray matter of the upper part of 

 the floor of the fourth ventricle and partly from a collection of nerve-cells situated 

 at the side of the aqueduct of Sylvius, from which the fibres pass downward under 

 the name of the descending root of the fifth. The real origin of the sensory root 

 is from the Gasserian ganglion, which corresponds with the ganglion on a spinal 

 nerve (see Development of Spinal Nerves in section on Embryology). 



The two roots of the nerve pass forward below the tentorium cerebelli as it 

 bridges over the notch on the inner part of the superior border of the petrous 

 portion of the temporal bone: they then run between the bone and the dura 

 mater to the apex of the petrous portion of the temporal bone, where the fibres 

 of the sensory root form a large semilunar ganglion (Gasserian), while the motor 



