204 TOPOGRAPHICAL ANATOMY OF THE 



formation of the floor of the fourth ventricle, and will be examined later. 

 On each side the pons is associated with the superficial origin of the 

 trigeminal nerve, and beyond this is continued into the cerebellum as 

 the hrachiam pontis. 



A superficial examination of the pons suffices to show that it is 

 composed largely of transverse fibres. The greater part of these are 

 superficial to the pyramids ; but some cross the dorsal aspect of these 

 bundles and, since they are more posterior than the superficial fibres, 

 form an elongated transverse area, the trapezoid body (corpus trapez- 

 oideum), on the surface of the brain. From the lateral part of this 

 body the facial and acoustic nerves have their superficial origin. 



Dissection. — With a very little dissection the course of the pyramids 

 through the pons can be demonstrated. Make a longitudinal incision 

 across the middle of the pons, and a similar incision just medial to the 

 origin of the trigeminal nerve. Then, with a pair of forceps, tear away 

 the superficial transverse fibres of the pons until the pyramid is exposed. 

 In this way the continuity of the pyramid with the basis pedunculi can 

 be determined. 



The Cerebelium. — The cerebellum, a transversely elongated rounded 

 mass, forms the dorsal and more bulky part of the hind-brain. For 

 descriptive purposes three parts may be distinguished — namely, a 

 central vermis and two hemispheres. The vermis is the most prominent 

 part of the organ, and projects beyond the level of the hemispheres as a 

 rounded ridge, more sharply defined in its posterior and ventral parts 

 than elsewhere. The hemisphere, lateral in position, is narrow anteriorly 

 but rapidly widens as it is followed backwards. Since the restiform 

 body, the brachium pontis and the brachium conjunctivum — generally 

 designated collectively the cerebellar peduncles — are connected with 

 the hemisphere and not with the vermis, it is clear that the only mode 

 of access of fibres to the cerebellum is through the hemisphere. The 

 restiform body and the brachium pontis have been noted in connection 

 with the medulla oblongata and the pons respectively. The whole of 

 the brachiitrn conjunctivum ^ cannot be seen at present, but a glimpse 

 of it may be obtained between the brachium pontis and the posterior 

 colliculus of the corpora quadrigemina. 



The surface of the cerebellum is beset with narrow ridges or folia, 

 separated by fissures. Deeper fissures cut the organ into lobes, which 

 can be more easily studied in sections, as will be done presently. It 

 should be observed, however, that the most lateral part of the hemi- 

 sphere is isolated from the rest by a fissure running in a sagittal direction. 

 The portion so defined is composed of the paraflocculus and flocculus. 



^ Conjunciio [L.], a joining together, a union. 



