NERVOUS CENTRES. (HUMAN ANATOMY. THE ENCEPHALOU.) 



693 



are the peduncles of the cms cerebelli. Through 

 them the cerebellum forms a connection with 

 other parts of the encephalon. The superior 

 layer or peduncle is a bundle of fibres which 

 extends to the corpora quadrigemina, and may 

 be traced beneath them to the optic thalami. 

 These are the procesms cerebelli ad testes, but 

 from their being obviously a medium of con- 

 nection between the cerebellum and the cere- 

 brum, they may be better named cerebro-cere- 

 bellur commissures. It is worthy of remark, 

 that these are the only fibres which appear to 

 connect these two segments of the brain. The 

 middle layer is continuous with the restiform 

 bodies, processus cerebelli ad medulluw oblon- 

 galum. And the inferior layer is evidently de- 

 rived from the transverse fibres of the pons 

 Varolii, which thus pass from one hemisphere 

 to the other, and constitute a great commissure 

 to the cerebellar hemispheres. These fibres, 

 moreover, connect each hemisphere to the me- 

 socephale (fig. 380, t, r, v). 



From this triple constitution of the crus 

 cerebelli, it is plain that the cerebellum may 

 exert an influence upon, or be affected by the 

 optic thalami or quadrigeminal bodies, the 

 restiform columns, or the mesocephale. 



Of the fourth ventricle. This is a rhomboi- 

 dal cavity, situated at the upper and posterior 

 part of the medulla oblongata, and extending 

 over part of the superior surface of the meso- 

 cephale. It is limited superiorly by the poste- 

 rior margin of the testes, and inferiorly by the 

 superior blunt extremity of the posterior pyra- 

 mids. Its two lateral angles correspond to the 

 entrance of the restiform bodies into the crura 

 cerebelli. In fact, it is formed by the diver- 

 gence of the restiform columns in their ascent 

 to the hemispheres of the cerebellum. The 

 median lobe of the cerebellum lies over the 

 fourth ventricle, and conceals it from view. 

 The anterior lobule of the inferior vermiform 

 process, the nodule, projects into it, and 

 closes it below. On either side of this lobule 

 a process of pia mater, with small granulations 

 upon it, is found. These processes are the 

 choroid plexuses of the fourth ventricle. A- 

 round these and thence on to the nodule, the 



Fig. 392. 



Vertical section of the median lobe of cerebellum, mesocephale, 

 and medulla oblongata, to skew the fourth ventricle. 



o, corpus dentatum , f, posterior surface of medulla oblon- 

 gata ; p, pons Varolii ; </, processus cerebelli ad tcslesj v, 

 cavity of the fourth ventricle. 



proper membrane of the ventricle is reflected, 

 and thus its cavity is shut out from any com- 

 munication with the subarachnoid cavity. A 

 vertical section in the median plane, or a little 

 to one side of it, displays this arrangement 

 well. (Fig. 392.) 



Along the floor of the fourth ventricle we 

 find the central or olivary columns of the me- 

 dulla oblongata extending upwards to the optic 

 thalami. A fissure, continuous with the pos- 

 terior median fissure, separates these columns, 

 and terminates above in a canal which pene- 

 trates the mesocephale, to reach the third ven- 

 tricle : iter a tertio ad quurtum ventriculum or 

 aqutducf of Sylvius. On either side of the 

 fissure certain bundles of white fibres, conti- 

 nuous with the auditory nerves, join it at right 

 angles, crossing over the olivary columns. This 

 fissure, with its white fibres on each side, lias 

 been compared to a pen with its barbs, and 

 hence called calamus scriptorius. 



The fourth ventricle, although sometimes 

 called the ventricle of the cerebellum, properly 

 belongs to the medulla oblongata. It is present 

 in all the vertebrate classes, and in size bears 

 a direct proportion to that of the medulla 

 itself. 



Op THE HEMISPHERES OF THE BRAIN. 



A mass of fibrous matter, covered on its ex- 

 terior by a convoluted layer of vesicular matter, 

 inflected towards the mesial plane above and 

 below a pair of gangliforrn bodies, (uptic thu- 

 luini and corpora xtriuta,) which it thus encloses 

 in a cavity or ventricle this, with certain fibres 

 connecting its anterior to its posterior parts, 

 forms a cerebral hemisphere. The hemispheres 

 of opposite sides are applied to one another 

 along the mesial plane, leaving the fissure-like 

 interval called the third ventricle; and they are 

 united by a plane of transverse fibres, the 

 greater part of which is placed above that ven- 

 tricle, but which bends down anteriorly as well 

 as posteriorly, closing the fissure at those 

 situations. 



Of the convolutions. That which first at- 

 tracts attention in connection with the cerebral 

 hemispheres, as affording the highest phy- 

 siological as well as anatomical interest, is 

 their convoluted surface. This can 

 only be well displayed by strip- 

 ping off the pia mater. The ap- 

 pearance which is then presented 

 has been variously described by 

 different writers. It has always 

 seemed to me to resemble the folded 

 surface formed by the mucous mem- 

 brane of the stomach when the mus- 

 cular coat is very much contracted. 

 The rugae of that membrane be- 

 come enormously developed by the 

 excessive contraction of the mus- 

 cular coat : the mucous membrane 

 not possessing any contractile power 

 is thrown into thin folds to adapt 

 it to the diminished capacity of the 

 stomach. Its folded state indicates 

 a great disproportion between the 

 extent of the mucous surface and 

 that of the muscular tunic. If both 



