HEAD AND NECK OF THE HORSE 205 



The parafioGculus ^ consists of a double row of folia, and into the depths 

 of the fissure separating it from the main mass of the hemisphere the 

 brachium pontis disappears. 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 some depth. 



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

 the cerebellum and the two medullary vela (thin membranous structures 

 forming the roof of the fourth ventricle). By separating the two halves 

 of the cerebellum 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. 



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, 

 while the white matter within, as a consequence of the presence of 

 numerous fissures, presents a tree-like appearance in section, and there- 

 fore goes by the name of arbor vitce." From the central mass of white 

 matter (corpus medullare) various branches (lamina? medullares) radiate. 

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

 largest lobes separated by the deepest of the cerebellar fissures. 



Two of the connections of the cerebellum — namely, the restiform 

 body and the brachium pontis, have previously been examined, but up 

 to the present, the third connection — the hrachiiimi conjunctivum — 

 could not be seen in its whole length. If now the two halves of the 

 cerebellum be separated as much as possible, the brachium will be 

 disclosed at its connection 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. At the point of their disappearance, the 

 superficial origins of the trochlear nerves should be noted. 



Ventral to the cerebellum, and connected round the ends of the 

 vermis with the white matter of this organ, are two thin, nervous 

 membranes that form the greater part of the roof of the fourth ventricle. 

 The anterior medullary veluin'^ (velum medullare anterius) is a 

 triangular membrane that fills the gap between the two brachia con- 

 junctiva. The base of the triangle bends round the end of the vermis 

 and is continuous with its white core. The posterior rtiedullary velum 

 (velum medullare posterius) is somewhat more complicated. Its 

 cerebellar connection stretches from the median plane — where it has 



^ vapd (para) [Gr.], beside. Flocculus (dim. oijioccus) [L.], a Httle tuft or flock 

 of wool. 



2 Arhor [L.], a tree. Vita [L.], life. 



^ Velum [L.], a covering, an awning, a veil. 



