COURSE OF THE ASCENDING FIBRES THROUGH THE PONS. 579 



has been called the commissure' of the cerebellum, are seen in 

 the fetus, to form a junction with the ascending fibres of the 

 corpus pyramidale. 



The fibres composing the anterior half of the lateral column 

 (corpus innominatum) ascending principally to the outer side 

 or back part of the corpus olivare, plunge into the pons varolii, 

 from the floor of the fourth ventricle, and pursue their course 

 through the crus cerebri, F, fig. 224, to their appropriate gan- 

 glion in front, the posterior cerebral, o, o, better known under 

 the name of thalamus nervi optici. These fibres form the 

 upper part of the crus cerebri, and are separated in that 

 body, from the ascending fibres of the corpus pyramidale, by 

 a deposit of dark-colored cineritious matter, called the locus 

 niger of Soernrnering. This tract, which is considered sensory 

 by Bell and Solly, and motory by Foville,* is covered above 

 where it forms the upper part of the crus cerebri by the 

 valve of Vieussens, and the tubercula quadrigemina g. The 

 course of these fibres through the posterior cerebral ganglion 

 (thalamus) is not so distinctly marked, as those of the corpus 

 pyramidale J5, fig. 224, through the corpus striatum, or ante- 

 rior cerebral ganglion ; they are more intimately intermixed 

 with the cineritious substance in the centre of the former. 

 From the outer side of this ganglion, the medullary fibres are 

 seen issuing forth, and spreading in every direction to terminate 

 in the cineritious outer covering of the cerebrum. The pons, it 

 will be seen then, as its name imports, is a sort of bridge 

 between the medulla oblongata, cerebrum, and cerebellum ; 

 formed of transverse fibres coming from the cerebellum particu- 

 larly well marked on the lower surface of the pons, of the 

 ascending fibres of the medulla oblongata, and of the processus 

 e cerebella ad testes or oblique commissure which passes on its 

 upper surface from each lobe of the cerebellum to the tubercula 

 quadrigemina, bodies which are placed on the back of the pons. 

 Included within these parts immediately below the valve of 

 Vieussens which is spread between the two processes or oblique 

 commissures last mentioned, is the fourth ventricle, or ventricle 

 of the cerebellum, from which a canal called the aqueduct of 

 * Encephalon, Diet, de Med. et de Chir. Pratiques. 



