734 THE NERVOUS SYSTEM. 



crescentic lobule above and flocculus below and its fibres contribute to form part 

 of the white matter of the hemisphere. 



The inferior peduncle, restiform body of medulla, as it enters the cerebellum 

 lies a little deeper in the anterior notch than, and inferior to, the middle pedun- 

 cle. Within the cerebellum its fibres blend with those of the preceding to form 

 the white matter of the hemisphere. Just as this peduncle bends sharply back- 

 ward from the medulla, and just before it actually enters the hemisphere, its under 

 surface is free, and forms, in this situation, the upper boundary of the cleft, above 

 referred to, between the layers of the white matter of the cerebellum. The lower 

 boundary of this part of the cleft is the lateral part of the inferior medullary velum. 



Inferior Medullary Velum (Figs. 426, 429, 432, 433). As already mentioned, 

 this is the lower layer of the white matter of the cerebellum, and is very thin. Its' 

 central portion enters the cerebellum over or dorsal to the nodulus at the centre of 

 the anterior notch ; and at the bottom of this part of the great horizontal fissure 

 it joins with the superior medullary velum to enter the cerebellum as the white 

 matter of the worm. As it passes over the nodulus it is adherent to it i. e. it 

 sends into the nodule a stem of white matter. 



As the velum passes laterally it has a curved direction, concavity forward, and 

 extends almost to the limits of the anterior notch, where it blends with the Avhite 

 matter of the flocculus. These curving portions, lateral to the nodulus, are the 

 so-called lateral parts of the velum. Each of these lateral parts, traced into the 

 horizontal fissure, passes above or dorsal to the peduncle of the flocculus (see above), 

 and blends with the under surfaces of the three peduncles to form the white matter 

 of the hemisphere. Thus it is seen that the white matter of the worm is made up 

 of the union of the superior medullary velum above and the central part of the 

 inferior medullary velum below, while that of the hemispheres is the conjoined 

 three peduncles and lateral part of the inferior velum. 



Tent and Lateral Recess. The cleft between these layers of white matter is, 

 like them, divisible into a central and two lateral portions. The central part lies 

 between the superior velum above and the central portion of the inferior velum 

 below. It is called the tent, from its pointed appearance on section. The lateral 

 portions, when closed in by the upper ends of the restiform bodies (see page 738), 

 are known as the lateral recesses of the fourth ventricle, while the tent forms the 

 roof of the central part of the fourth ventricle (see page 738). 



Superior Medullary Velum. The superior medullary velum, or valve of Vieus- 

 sens, has been partially described. It is a thin lamina of white matter stretched 

 between the inner margins of the superior cerebellar peduncles ; it forms the roof 

 of the upper half of the fourth ventricle. It is narrow above, where it passes 

 beneath the lower corpora quadrigemina (mid-brain), and broader below at its 

 connection with the upper vermiform process of the cerebellum. A slight elevated 

 ridge, the frcenulum, descends upon the upper part of the valve from between the 

 lower corpora quadrigemina, and on either side of it may be seen the fourth nerve. 

 Its lower half is covered by a thin, transversely-grooved lobule of gray matter 

 prolonged from the anterior border of the cerebellum ; this is the lingula. 



Arbor Vitse (Figs. 428, 429). This is the name given to the white matter of 

 either worm or hemisphere when viewed on antero-posterior section. On such a 

 section the white matter looks like a tree with a central trunk and branches, with 

 the branches also subdividing into stems. These stems, being surrounded by gray 

 matter, resemble leaves or folia ; and there may also be secondary folia whose 

 stems come from a primary stem, and not from the main trunk of white matter. 

 These folia, as already explained (page 728), are merely the cut surfaces of the 

 corresponding lobules, whether of worm or hemisphere. The main trunk of the 

 arbor vitae of the worm is slender, while that of the arbor vitae of each hemisphere 

 is thick and bulky. This difference is due to the large amount of white matter 

 resulting from the conjoined peduncles and lateral part of inferior velum as com- 

 pared with that resulting from the union of the comparatively thin superior velum 

 with the central part of the inferior velum. 



