MEDULLARY STRUCTURE OF THE CEREBELLUM. 575 



— The medullary matter contained in the leaflets of each lobule, 

 are radiations from a larger medullary mass at the base of the 

 lobule, called a twig. The medullary twigs of each lobe, are 

 derived in a similar way from larger white masses, called branches, 

 and the branches again form a large medullary mass in the cen- 

 tre of each lobe called the trunk. The whole of this beautiful 

 arrangement of the medullary matter, forms what is called fanci- 

 fully, the thuya, or arhor vita. If we trace these medullary 

 fibres again out in a reversed direction (that is from the centre of 

 each lobe of the cerebellum, towards the pons and medulla ob- 

 longata,) we shall find that anteriorly the trunk of the arbor vitae 

 or the medullary nucleus, advances towards the medulla oblongata, 

 inclosing laterally the fourth ventricle ; each trunk then divides 

 into three processes or pedunciiU ; one pair passes up as rounded 

 columns to the testes of the tubercula quadrigemina, and is 

 called the peduncle of these bodies, processus e cerebello ad 

 testes oblique or intercerebral commissure of Solly ; a second 

 pair to the medulla oblongata, (corpora restiformia,) and a third 

 pair, over the lower surface of the pons, called its peduncle or 

 the great commissure of the cerebellum. Under the term crus 

 cerebelli, all these three parts are commonly, though inaccurately 

 comprised. Between the inner borders of the two rounded col- 

 umns which go to the testes, and closely adherent to them, is 

 spread a thin layer called the valve of Vieusseus, valve of the 

 brain, valve of the cerebellum.. This so called valve, forms the 

 roof to the fourth ventricle ; when it is broken through, that 

 cavity is exposed. 



— Each one of the lamina or leaflets of the lobes of the cere- 

 bellum, are susceptible of being unfolded in a properly prepared 

 part, and spread out so as to form a flat membrane, with a cine- 

 ritious covering on its outer side, and a medullary layer within, 

 consisting of parallel fibres, which can be traced down to the 

 central medullary twig of each lobule. The last part of the 

 stratum consists of coarse and curvilinear fasciculi, which con- 

 stitute the central mass of medullary matter of each hemisphere 

 (trunk of the arbor vitse,) fibres of which may be raised up in a 

 properly prepared cerebellum with the forceps, in fine filaments 

 the size of the thread of a silkworm. 



