CEREBELLUM. 531 



The median lobe is the fundamental part, and in some ammaiS, as fishes and reptiles, 

 the only part which exists; the hemispheres being additions, and attaining their 

 maximum in man. The hemispheres are separated, in front, by a deep notch, the 

 incisura cerebelli anterior, which encircles the corpora quadrigemina behind ; they 

 are also separated by a similar notch behind, the incisura cerebelli posterior, in 

 which is received the upper part of the falx cerebelli. The superior vermiform 

 process (upper part of the median lobe of the cerebellum), extends from the notch 

 on the anterior to that on the posterior border. It is divided into three lobes : the 

 lobulus centralis, a small lobe, situated in the incisura anterior ; the monticulus cere- 

 belli, the central projecting part of the process ; and the commissura simplex, a 

 small lobe near the incisura posterior. 



The under surface of the cerebellum (fig. 271) is subdivided into two lateral 

 hemispheres by a depression, the valley, which extends from before backwards in 

 the middle line. The lateral hemispheres are lodged in the inferior occipital fossae ; 

 the median depression, or valley, receives the back part of the medulla oblongata, 

 is broader in the centre than at either extremity, and has, projecting from its floor, 

 part of the median lobe of the cerebellum, called the inferior vermiform process. 

 The parts entering into the composition of this body are, from behind forwards, the 



Fijr. 271. Under Surface of che Cerebellum. 



commissura brevis, situated in the incisura posterior ; in front of this, a laminated 

 conical projection, the pyramid; more anterior, is a larger eminence, the uvula, 

 placed between the two rounded lobes which occupy the sides of the valley, the 

 amygdalae, and connected with them by a commissure of gray matter, indented 

 on the surface, called the furrowed band. In front of the uvula is the nodule; 

 it is the anterior pointed termination of the inferior vermiform process, and projects 

 into the cavity of the fourth ventricle; it has been named by Malacarne the 

 laminated tubercle. On each side of the nodule is a thin layer of white substance, 

 attached externally to the flocculus, and internally to the nodule, and to a corre- 

 sponding part on the opposite side ; they form together the posterior medullary 

 velum or commissure of the flocculus. It is usually covered in and concealed by 

 the amygdalae, and cannot be seen until these are drawn aside. This band is 

 of a semilunar form on each side, its anterior margin being free and concave, its 

 posterior being attached just in front of the furrowed band. Between it and the 

 nodulus and uvula behind, is a deep fossa, called the swallow's nest (nidus hirun- 

 dinis]. 



Lobes of the cerebellum. Each hemisphere is divided into an upper and a lower 

 portion by the great horizontal fissure, which commences in front at the pons, and 

 passes horizontally round the free margin of either hemisphere, backwards to the 

 middle line. From this primary fissure numerous secondary fissures proceed, which 

 separate the cerebellum into lobes. 



