PATHS OF THE CRANIAL NERVES 551 



of the trigeminus, and through this nerve trunk are distributed to 

 the muscles of mastication. 



The sensory root of the trigeminus, larger than the motor, 

 enters the lateral portion of the pons Varolii slightly caudad to the 

 motor root. Its fibres pass in a dorso-lateral direction until near 

 the grey matter, where they divide into very short ascending and 

 very long descending bundles. The latter form the long spinal 

 root of the trigeminus, which is continued downward on the inner 

 side of the restiform body to the cervical region of the spinal cord. 

 Its fibres and collaterals successively end about the small nerve 

 cells contained in the adjacent substantia gelatinosa of Rolando, 

 which is continued upward from the tips of the dorsal horns in the 

 spinal cord. The neuraxes of these cells sensory trigeminal neu- 

 rones of the second order decussate as internal arcuate fibres and 

 pass cerebralward in the mesial fillet. 



The short ascending branches, together with many collaterals 

 from the descending divisions, end in the chief sensory nucleus of 

 the trigeminus, which begins in the extreme lateral portion of the 

 pontal grey matter somewhat cephalad from the trigeminal root, 

 and by a tapering extremity is continued spinalward as far as the 

 gelatinous substance of Rolando in the medulla oblongata, with 

 which portion of the grey matter it appears to be continuous. 



The path of the central neurones from this nucleus is still un- 

 certain. They probably pass, after the mariner of the internal 

 arcuate fibres, to the mesial fillet of the opposite side. 



The Fourth or Trochlear Nerve (Figs. 388 and 389). The 

 peripheral neurones of the trochlear nerve begin in the large motor 

 nerve cells of the trochlear nucleus, which lies in the grey matter 

 between the aqueduct of Sylvius and the posterior longitudinal 

 fasciculus at the level of the cephalic border of the inferior corpora 

 quadrigemina and the decussation of the superior cerebellar pedun- 

 cles. The nucleus comprises a compact group of large stichochrome 

 nerve cells, which are in close relation to the dorsal surface and lat- 

 eral margin of the posterior longitudinal fasciculus. These nuclei 

 are characteristically asymmetrical in size, shape, and position. 



From the nerve cells of this nucleus neuraxes pass spinalward 

 in a small compact bundle lying in the ventro-lateral angle of the 

 grey matter surrounding the aqueduct. This descending root of the 

 trochlear nerve is placed dorso-mesial to the descending or mesen- 

 cephalic root of the fifth nerve, and dorso-lateral to the posterior 

 longitudinal fasciculus. 



