Chap. 27.] fbe Cerebellum. 273 



into, to contain a gravelly fubftance, which refifts the 

 knife. Below the pineal gland there is a tranfverfe me- 

 dullary cord, called the pofterior commilTure of the 

 hemifpheres of the cranium. 



The cerebellum is fituated under the tranfverfe fep- 

 tum, in the pofterior and lower part of the cranium. 

 Like the cerebrum it is compofed of cineritious and 

 medullary matter. It differs from the cerebrum, how- 

 ever, in having no circumvolutions on its furface ; in- 

 fcead of thefe, we here obferve numerous furrows run- 

 ning parallel to each other, and nearly in a tranfverfe 

 direction, into which enter folds of the pia mater. 

 Under the tranfverfe fepium it is covered by a vafcular 

 texture which communicates with the plexus choroides. 

 It has four eminences which are turned in different di- 

 rections, and which from fome refemblance to the rings 

 of an earth-worm are named appendices vermiformes. 

 The cerebellum is divided 'into two lateral parts by 

 the fmall falx ; on the back part it is divided into 

 two lobes feparated by che occipital feptum of the dura 

 mater. 



By cutting deeply into the fubftance of the cerebel- 

 lum in the direction of its falx, \ve obferve an oblong 

 cavity which is called the fourth ventricle ; this termi- 

 nates Backwards like the point of a writing-pen, and 

 this end of it has therefore been called calamus fcrip- 

 torius. At the beginning of this cavity we meet 

 with a thin medullary lamina which has been confldered 

 as a valve. This ventricle is lined like the others with 

 the pia mater, which is continued through all thefe 

 cavities. 



The fubftance of the cerebellum appears very dif- 

 ferent, according to the direction in which it is cut. 

 By dividing it vertically we find the medullary part 

 difpofed fo as to exhibit ' the appearance of a tree. 

 VOL. III. T Thcfe 



