DISSECTION OF THE DOG 225 



the petrous portion of the temporal bone. Into the depths of the fissure 

 between the paraflocculus and the rest of the hemisphere the brachium pontis 

 can be followed. Between the paraflocculus and the origins of the facial and 

 acoustic nerves is the flocculus, a small collection of folia separated from the 

 paraflocculus by a fissure of considerable depth. 



Dissection. — Make a median sagittal section through the vermis of the 

 cerebellum and the two medullary vela — thin membranes forming the 

 roof of the fourth ventricle. By separating the two halves of the cere- 

 bellum a view of the floor of the ventricle may be obtained. The cut 

 surface of the vermis should be examined and the arrangement of 

 the lobes and fissures noted. 



Arbor vitje. — A sagittal section of the vermis of the cerebellum reveals 

 in a very striking manner the peculiar arrangement of grey and white matter. 

 The grey matter forms a continuous layer on the surface of the organ. The 

 white matter within, as a consequence of the presence of numerous fissures, 

 presents a tree-like appearance in sections, and therefore goes by the name of 

 arbor vitae. From the central mass of white matter various branches radiate. 

 Of these, two are conspicuously large and form the core of the two largest 

 lobes of the cerebellum separated from each other by the deepest of the 

 cerebellar fissures. 



Brachium conjunctivum. — Two of the connections of the cerebellum, viz. 

 the restiform body and the brachium pontis, have previously been examined. 

 Up to the present, however, the third connection — the brachium conjunctivum — 

 could not be examined in its whole length. If the two halves of the cerebellum 

 be separated as much as possible the brachium will be seen connected with the 

 hemisphere immediately within the brachium pontis. Followed towards 

 the mid-brain, each brachium runs in a slightly medial direction to disappear 

 under the posterior colliculus of the corpora quadrigemina. 



Medullary vela. — Ventral to the cerebellum, and connected round 

 the ends of the vermis with the white matter of this organ, are two thin mem- 

 branes which form the roof of the fourth ventricle. The anterior medullary 

 velum (velum medullare anterius) is a triangular membrane filling the space 

 between the two brachia conjunctiva. The base of the triangle bends round 

 the end of the vermis and is continuous with its white core. 



The posterior medullary velum (velum medullare posterius) is somewhat 

 more complicated. Its cerebellar attachment stretches from the median plane 

 where it has a connection similar to that of the anterior velum — to the floc- 

 culus. The velum soon loses its nervous elements and gives place to a very 

 thin translucent membrane composed of pia mater lined internally with 

 ependyma. This completes the posterior part of the roof of the fourth 

 ventricle and is attached to the margins of .the cavity. 



Ventriculus quartus. — The fourth ventricle represents the cavity of 



