224 DISSECTIOX OF THE DOG 



with a very shallow anteroposterior groove in the middle line. The dorsal 

 surface assists in the formation of the floor of the fourth ventricle, and will be 

 examined later. On each side the pons is continued into the cerebellum as 

 the brachium 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. Some, however, cross the dorsal aspect of the pyramids, and, 

 since they are more caudal than the superficial fibres, form an elongated 

 transverse area, the trapezoid body (corpus trapezoideum) on the surface of 

 the brain. From the lateral part of this body the facial and acoustic nerves 

 have their superficial origin. 



Dissection. — The course of the pyramids should be followed through the 

 pons. This can be done by making a longitudinal incision across the 

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

 trigeminal nerve. Now, with a pair of forceps, tear away the superficial 

 transverse fibres of the pons until the pyramid is exposed. The con- 

 tinuity of the pyramid with the basis pedunculi can thus be demonstrated. 



Cerebellum. — 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 caudal and ventral parts than elsewhere. The hemisphere, lateral in 

 position, is narrow anteriorly but rapidly widens. A small lateral appendage 

 is conspicuous when the brain has been removed from the cranium, and is 

 received into a depression in the temporal bone when the parts are in their 

 natural position. It is with the hemisphere that the restiform body, the 

 brachium pontis, and the brachium conjunctivum are connected ; that is to 

 say, access to the vermis is only obtained by way of the hemisphere. The 

 restiform body and the brachium pontis have been mentioned in connection 

 with the medulla oblongata and pons respectively. At the present stage of 

 the dissection the brachium conjunctivum cannot be seen in its entirety, 

 but a glimpse of it is 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 cerebellum into lobes, the disposition of 

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

 should be observed, however, that the most lateral part of each hemisphere is 

 separated from the rest by a fissure running in a sagittal direction. The 

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

 flocculus consists of two rows of folia from the more ventral of which projects 

 the petrous lobule (lobulus petrosus), so named from its location in a fossa of 



