1596 A MANUAL OF ANATOMY 



cerebellar hemisphere the following parts are to be noted : (i) the postero- 

 inferior lobule, composed of (a) the lobulus gracilis, and {b) the inferior semi- 

 lunar lobule ; (2) the biventral lobule ; (3) the amygdala ; and (4) the flocculus. 



The dissectors should also make themselves familiar with the peduncles 

 of the cerebellum, namely, the superior, middle, and inferior peduncles, right 

 and left. The superior peduncles will be seen to converge towards the lower 

 pair of quadrigeminal bodies, after they emerge from the cerebellar hemi- 

 spheres, the valve of Vieussens having been already observed extending 

 between them. The middle peduncles, formed by the transverse fibres of 

 the pons Varolii, sink into each cerebellar hemisphere. The inferior peduncles, 

 which constitute the restiform bodies, connect the cerebellar hemispheres 

 with the upper or open part of the medulla oblongata posteriorly. 



Fourth Ventricle. — This ventricle is to be laid open by making a median 

 incision through the vermis of the cerebellum from above. This will bring 

 into view the central collection of white matter called the corpus trapezoides. 

 The two parts of the cerebellum are then to be gently separated, so as to 

 expose the superior medullary velum, or valve of Vieussens, the inferior 

 medullary velum, and the tela choroidea inferior, which lies lower down than 

 the inferior medullary velum, and presents in the median line the foramen of 

 Magendie. The obex is to be looked for in the lower part of the roof, where 

 it overhangs from behind the upper opening of the central canal of the spinal 

 cord, and on either side the ligula may be seen projecting over the epi- 

 thelium of the roof for a short distance. When these structures have been 

 divided, the floor of the fourth ventricle will be exposed. Its rhomboidal 

 shape is to be noted. At the lower end the dissectors will observe the open- 

 ing of the central canal of the spinal cord and the calamus scriptorius, and 

 at the upper end the lower opening of the aqueduct of Sylvius. The longi- 

 tudinal groove, traversing the floor between these two openings, is to be 

 noted, and the striae acusticse crossing the floor towards its centre, on either 

 side of this groove, are to receive attention. The floor is thus divided into two 

 portions, lower and upper. The lower or medullary portion will be seen to 

 be bounded on either side, from below upwards, by the clava, the cuneate 

 tubercle, and the restiform body, which latter should now receive full con- 

 sideration. The parts to be noted in connection with each half of the lower 

 portion of the floor are as follows : (i) the fovea inferior, (2) the trigonum 

 vagi, (3) the trigonum hypoglossi, and (4) the trigonum acusticum. The 

 trigonum acusticum may be seen to be continued superiorly into a promi- 

 nence called the tuberculum acusticum, over which the striae acusticae pass. 



The upper or pontine portion of the floor will be seen to be bounded on 

 either side by the superior peduncle of the cerebellum, and the parts to be 

 noted in connection with each half are as follows : (i) the fovea superior ; 

 (2) the eminentia teres, produced by the subjacent fasciculus teres ; (3) the 

 locus cseruleus ; and (4) an upward prolongation of the tuberculum acusticum. 



The lateral recess of the fourth ventricle is to be examined. It will be 

 found as a lateral prolongation of the ventricle, on either side, round the upper 

 part of the restiform body. In the roof of each lateral recess is the foramen 

 of Kay and Retzius. 



The great horizontal fissure of one cerebellar hemisphere is now to be 

 opened up, and the upper half of the hemisphere is to be removed. This will 

 give the dissectors a view of the three cerebellar peduncles as they enter the 

 white medullary substance in the centre of the hemisphere. This substance 

 contains the corpus dentatum, which is a wavy lamma of grey matter en- 

 closing white matter, and open at the upper and inner part. Through this 

 opening many of the fibres of the superior cerebellar peduncle emerge. To 

 expose the corpus dentatum a cerebellar hemisphere has to be cut through 

 horizontally a little above the level of the centre. Sagittal sections are also 

 to be made across the direction of the cerebellar laminae, when the appear- 

 ance known as the arbor vitae cerebelli will become evident. 



Lastly, with proper assistance, sections should be made through the pons 

 Varolii, with the view of obtaining some knowledge of its internal structure. 



