THE FIFTH NERVE. 



233 



While lodged in the outer wall of the sinus, the fourth nerve is connected with 

 the sympathetic on the carotid artery, and is also joined by a filament from the 

 ophthalmic nerve. 



The fourth nerve consists of about 1,200 fibres, mostly of large size. It also shows close 

 to its origin the vestiges of a degenerated ganglion (Gaskell). 



Varieties. In one case the fourth nerve pierced the levator palpebras superioris on its 

 way to the superior oblique (G-. D. T.). The nerve has been observed in several cases sending 

 a branch forwards to the orbicularis palpebrarum muscle, or to join the supratrochlear, the 

 infratrochlear, or the nasal nerve. A communication with the frontal nerve is recorded by 

 Berte. 



V. TRIFACIAL NERVE. 



The fifth, trifacial, or trigeminal nerve is the largest of the cranial nerves, and 

 resembles a spinal nerve in the circumstance that it arises by separate sensory and 



Fig. 153. PLAN OP THE ORIGIN OF THE FIFTH NERVE. 



The outline represents the contour of the medulla ohlongata, 

 pons, and a part of the raidbrain, which are supposed to be 

 transparent : V.M., motor portion of the fifth nerve ; N.V.M., the 

 motor nucleus; V.D., descending or mesencephalic root; \ r .S., 

 sensory portion of the fifth nerve; N.V.S., the upper sensory 

 nucleus ; V.A., asoending or bulbar root ; S.GLR., gelatinous 

 substance of llolando, or lower sensory nucleus. 



motor roots, and also that the sensory fibres pass 

 through a ganglion while the motor do not. Its 

 sensory division, which is much the larger, imparts 

 common sensibility to the face and the fore part of 

 the head, as well as to the eye, the nose, the external 

 ear, and the mouth, including the greater portion of 

 the tongue ; it may possibly also confer the power of 

 taste upon the fore part of the latter organ. The 

 motor root supplies chiefly the muscles of mastication. 

 The two roots of the nerve appear at the side of 

 the pons Yarolii, where the transverse fibres of the 

 latter are prolonged into the middle peduncle of the 

 cerebellum, and much nearer its upper than its lower 

 border. The small root issues above the large one, 

 and the two are separated from one another by a small 



band of the cross fibres of the pons. The fibres of the small root arise in part from 

 the motor nucleus of the fifth nerve beneath the floor of the upper portion of the 

 fourth ventricle ; they are joined by the bundle known as the descending root of the 

 fifth nerve, which springs from a collection of large nerve-cells in the grey matter 

 at the side of the aqueduct of Sylvius. The fibres of the large root pass backwards 

 into the pons, and some reach the upper sensory nucleus of the fifth nerve, placed 

 to the outer side of and somewhat deeper than the motor nucleus ; but the greater 

 number turn downwards and are continued through the substance of the pons into 

 the medulla oblongata, forming the so-called ascending or bulbar root of the fifth 

 nerve : their mode of termination is uncertain, but they may break up among the 

 cells of the gelatinous substanca of Rolando (lower sensory nucleus of the fifth 

 nerve). 



The small root consists mainly of large fibres, and contains a vestigial ganglion (Gaskell). 

 The large root is mostly composed of fine fibres. 



