THE BRAIN AND ITS MEMBRANES. 733 



idal fissure. As it lies in the sulcus valleculae it separates the furrowed band 

 from a very slender lamina of gray matter which is continuous with the gray 

 matter of the nodule mesially, and, laterally, follows the course of the post- 

 nodular fissure until it reaches the flocculus, with the gray matter of which it is 

 continuous. This slender lamina is known as the peduncle of the flocculus. 



The flocculus is the smallest of the lobules of the inferior surface of the 

 hemisphere, and is situated farther away from its corresponding lobule of the 

 inferior worm than any of the others. It is a rounded, tuft-like body, its 

 expanded extremity looking forward, and it tapers toward its peduncle. It is 

 situated below the middle peduncle of the cerebellum ; its surface is composed of 

 gray matter, subdivided into a few small laminae ; it is sometimes called the 

 pneumogastric lobule, from being situated behind the pneumogastric nerve. 



It is thus seen that the flocculus, amygdala, and digastric lobule differ in 

 regularity, both of outline and position, from all the other lobules of the hemi- 

 sphere ; also that the prepyramidal fissure differs from the other interlobular fis- 

 sures in that it, as a whole, is almost "horseshoe" in shape, while they have a 

 generally transverse direction. 



WHITE MATTER OF THE CEREBELLUM. Traced from within the cerebellum, 

 all the white matter is found to emerge from between the edges of the great hori- 

 zontal Ussure. where that fissure lies in the anterior cerebellar notch (Figs. 432, 

 433, 434). It may be described (after removal of pons and medulla by cutting 

 close to the cerebellum) as consisting of two layers, an upper and a lower. In 

 other words, this white matter on emerging from the cerebellum may be said to 

 split into two diverging layers. The cleft-like space between these two layers 

 extends entirely across the anterior cerebellar notch, at the lateral extremities of 

 which the two layers are continuous. It has already been noted (see page 727), 

 that the edges of the great horizontal fissure, in close contact everywhere else, 

 are separated in the anterior notch. Hence the space between the layers might 

 be regarded as a part of the horizontal fissure lined with white matter. Of these 

 two layers, the upper is much the thicker and more substantial, the lower being 

 merely a thin, delicate white lamina. 



The upper layer is divisible into a mesial and two lateral portions, of which 

 the mesial is much thinner than the lateral. This mesial portion is the valve of 

 Yieussens or superior medullary velum. It is of uniform thickness from side to 

 side. On transverse section, close to the cerebellum, its width is seen to be about 

 equal to that of the upper worm. It has above it the lingula, together with the 

 central lobule resting on the lingula. The lateral portions increase in thickness 

 from within outward, so that the cut surface of each looks somewhat racket- 

 shaped. Each lateral portion occupies the side of the anterior notch, and is made 

 up of the three peduncles of the cerebellum, the handle of the racket-shaped sur- 

 face representing the superior peduncle, while the rounded, expanded head repre- 

 sents, externally, the middle, and, inferiorly and mesially, the inferior, peduncle. 



The lower layer is the inferior medullary velum. It is an exceedingly delicate 

 white lamina stretching from the white matter of one flocculus across the middle 

 line to the white matter of the other flocculus. These different subdivisions will 

 now be considered in detail. 



Peduncles of the Cerebellum (Figs. 426, 432, 433). The superior peduncles 

 (Fig. 430) are on the dorsal surface of the pons Varolii, as previously described, 

 diverging from each other from above downward. Each enters the corresponding 

 hemisphere of the cerebellum beneath the frcenulum and ala (Fig. 430), where its 

 fibres blend with those of the two other peduncles and a part of the inferior medul- 

 lary velum, to form the white matter of the hemisphere. The superior peduncles 

 form the lateral boundaries of the upper part of the fourth ventricle. 



The middle peduncles are large rounded bundles made up of most of the trans- 

 verse fibres of the pons, as already described. Each, bending dorsally from the 

 pons, enters the cerebellum between the edges of the horizontal fissure at the 

 lateral limits of the anterior notch i. e. between the ala and the edge of anterior 



