540 THE NERVOUS SYSTEM 



In the diencephalon the posterior longitudinal fasciculus appar- 

 ently ends in the nucleus of Darkschewitsch or nucleus of the poste- 

 rior longitudinal fasciculus. 



Though we have traced the course of this fasciculus from the 

 spinal cord cephalward, it must be borne in mind that it contains 

 fibres which run both cephalad and caudad, and that it is therefore 

 subject to both ascending and descending degeneration. 



In its course through the brain the posterior longitudinal fas- 

 ciculus receives many fibres from the sensory nuclei of the cranial 

 nerves, especially from those of the ocular group with which it is 

 in close relation. By these connections the several groups of ocular 

 nuclei are brought into intimate relation with one another as well 

 as with those nuclei of closely related function which occur in the 

 cervical region of the spinal cord. 



The cerebellum possesses at least three important association 

 paths, the one placing this organ in relation with the opposite infe- 

 rior olivary body through the cerebello-olivary tract, the second 

 connecting it with the opposite cerebral hemisphere through the 

 superior cerebellar peduncles, and the third placing the cerebellum 

 in communication with the medulla oblongata and midbrain through 

 the middle cerebellar peduncles. 



The cerebello-olivary tract arises from nerve cells in the vermis 

 cerebelli, and, passing caudad through the restiform body, enters 

 the medulla oblongata. Its fibres here turn sharply inward and 

 ventralward in small discrete bundles and penetrate the formatio 

 reticularis, lying more lateral but parallel to the internal arcuate 

 fibres. Most of the bundles decussate to the opposite side and enter 

 the inferior olivary body, many of them first traversing the surface 

 of this nucleus. The fibres which thus surround the inferior olive 

 form for it an encapsulating sheath of medullated nerve fibres. A 

 few fibres of the cerebello-olivary tract end without decussation in 

 the inferior olivary body of the same side. 



The superior cerebellar peduncle (jbrachium conjunctivum) takes 

 origin from the cells of the vermis cerebelli and enters the mesen- 

 cephalon as a large round bundle of fibres which, at first, forms the 

 lateral roof of the fourth ventricle, but gradually sinks ventral- 

 ward, thus passing obliquely around the lateral border of the grey 

 matter which surrounds the aqueduct of Sylvius. The bundle 

 finally makes a sharp mesial curve toward its decussation at the 

 cephalic end of the mesencephalon. Its position is thus mesial and 

 dorsal to that of the mesencephalic portion of the fillet. 



