688 THE CENTRAL AXIS OF TEE NEBVOVS SYSTEM. 



their inferior part that the peduncles enter the substance of the cerebellum ; 

 and behind this point, beneath their lateral parts, the cerebellar plexus clioroides 

 is applied. 



The cerebellar choroid plexuses. — This name is given to two small reddish 

 granular masses, formed of vascular loops, elongated from before to behind, 

 depressed above and below, and comprised at their internal borders between 

 the corpora restiformia and the inferior face of the lateral lobes of the 

 cerebellum, to which they are strongly adherent by their superior face. 

 These two plexuses are joined together by means of Renault's valve, which 

 is united to them towards its base. 



Sulci and lobules of the cerebellum. — On examining, in a general 

 manner, all the sulci which intersect the external surface of the cerebellum, 

 we see that they penetrate to very unequal depths in the substance of the 

 organ, and that they divide it into successively decreasing segments, of 

 which Figs. 324 and 327 may furnish a sufficient idea. 



There is at first a certain number of principal lobules, which are divided 

 into secondary lobules ; and these, again, are in their turn separated into short 

 lamellse, representing the extreme limits of cerebellar lobulation. 



2. Internal Conformation and Structure of the Cerebellum. 



The cerebellum concurs, by its inferior plane and the internal face of 

 its peduncles, to form the cavity already described as the posterior or cere- 

 bellar ventricle ; but in the mass of the organ itself there is no trace of 

 excavation or other peculiarity. This is demonstrated in the most evident^ 

 manner by sections of its substance made either in an antero-posterior 

 or in a transverse direction. We only see in these traces of the sulci which 

 divide the organ into lobules ; and they also afford evidence as to the 

 structure of the cerebellum, showing that, like all the other parts of the 

 cerebro- spinal axis, it is formed of white and grey substance. 



The latter, spread over the entire surface of the organ, constitutes the 

 cortical layer of the different segments of which it is composed. It is even 

 prolonged into the convolutions which increase the surface-extent of the 

 cerebellum ; in each lobule it may be decomposed into superposed layers, 

 parallel to the lamina of white substance that forms the nucleus of the 

 lobule ; between these layers of grey substance is a very thin mass of white 

 matter. 



The white substance, enveloped on every side by the grey, forms two 

 thick nuclei occupying the centre of the lateral lobes, and which are united 

 and confounded on the median line in the texture of the middle lobe. 



These two nuclei, in continuity on each side with the cerebellar 

 peduncles, are only their prolongations or intercerebellar portions. They 

 send into the middle of each principal lobule a long and thiclj branch, which 

 gives off smaller divisions that ramify in the secondary lobules, and from 

 which escape a new series of ramuscules that enter the smallest segments ; 

 this gives to the cerebellum a beautiful arboreal aspect, justly designated 

 by the older anatomists the arbor vitce. (See Figs. 824, 327, 329, for 

 representations of the arbor vitcB cerebelli.) 



In the interior of these nuclei, a little in front, there sometimes exists a 

 small, shghtly-grey spot ; this is the trace of the corpus rhomboideum (or 

 dentatum of Man). 



The nuclei of the white substance of the cerebellum are constituted, like 

 the matter of the medulla, by nerve-tubes which are continuous on one side 



