1462 A MANUAL OF ANATOMY 



gives rise to the vermis or middle lobe of the cerebellum, and the lateral parts 

 give rise to the cerebellar hemispheres, or lateral lobes. 



The surface of the cerebellum is originally smooth and unbroken, but 

 fissures or sulci afterwards appear. The earliest of these is the floccular 

 fissure, of which there are two — right and left. At a later period many- 

 other fissures make their appearance, which impart a laminated character to 

 the lateral hemispheres and vermis. The floccular fissure of either side cuts 

 off the dorsal part of each lateral cerebellar plate (lateral cerebellar hemi- 

 sphere), and this part constitutes the flocculus. The parafiocculus on either 

 side is developed from the lateral part of each lateral cerebellar plate, close 

 to the rhombic lip. The two floccular fissures meet across the anterior part 

 of the inferior vermis, giving rise to the post-nodular fissure, which cuts off 

 the nodule from the anterior part of the inferior vermis. 



The flocculus and nodule are developed from the rhombic lip — the floccu- 

 lus from the lateral part and the nodule from the median part. 



The cephalic portion of the roof-plate of the metencephalon, close to the 

 isthmus, gives rise to the superior or anterior medullary velum {valve of Vieus- 

 sens), and lower down it forms the inferior or posterior medullary velum, 

 which is continuous with the ependymal or epithehal roof of the bulbar part 

 of the fourth ventricle. 



On either side of the valve of Vieussens the roof-plate is thickened by the 

 superior cerebellar peduncles. 



The superior peduncles of the cerebellum, right and left, arise from the cells 

 of the nucleus dentatus of the corresponding lateral cerebellar hemisphere. 

 Emerging from the anterior parts of the dentate nuclei, the peduncles give 

 rise to two thickenings of the roof -plate of the metencephalon, one on either 

 side of the valve of Vieussens. Thereafter they enter the mesencephalon or 

 mid-brain, and after decussating, each passes to the corresponding red nucleus 

 of the tegmentum of either crus cerebri, which constitutes its lower cell- 

 station. 



The middle peduncles of the cerebellum [brachia pontis) are developed 

 from the cells of the pontine nuclei of the pons Varolii, right and left. The 

 fibres of each peduncle issue from the lateral portion of the corresponding 

 pontine nucleus and enter the adjacent cerebellar hemisphere. 



The inferior peduncles of the cerebellum or restiform bodies are very com- 

 plex, inasmuch as they derive their fibres from several sources. These sources 

 are as follows : 



1. The olivo-cerebellar fibres of the Inferior olivary nucleus of the opposite 



side. 



2. The direct cerebellar tract of the lateral column of the spinal cord of 



the same side. 



3. The anterior superficial arcuate fibres from the nucleus gracilis and 



nucleus cuneatus of the opposite side. 



4. The posterior superficial arcuate fibres from the nucleus gracilis and 



nucleus cuneatus of the same side. 



5. Vestibular fibres from the vestibular nuclei of the vestibular division 



of the auditory nerve. 



Mesencephalon. — The mesencephalon represents the third secondary cerebral 

 vesicle, which also constitutes the middle primary cerebral vesicle. Its 

 thickened walls give rise to the parts around the aqueduct of Sylvius — 

 namely, (i) the crura cerebri, (2) the posterior perforated space, and (3) the 

 corpora quadrigemina. Its cavity, very much diminished, persists as the 

 aqueduct of Sylvius, or aqueductus cerebri. 



The crura cerebri and posterior perforated space are developed from the 

 ventral wall or floor of the mesencephalon. 



The alar plates of the lateral walls encroach so much upon the roof-plate 

 as to reduce it to a mere line. It presents a median longitudinal groove, 

 which separates two rounded eminences, known as the corpora bigemina. At 

 a later period a transverse groove appears, which divides each of the corpora 

 bigemina into two, thus giving rise to the corpora quadrigemina. 



