The posterior cerebellar notch, incisura ceredelli posterior (Fig. 'of), bounded by the posterior end of the worm and the postero-medial border of the hemispheres, is occupied by the falx cerebelli. A prolongation of the medullary body of the cerebellum fills up the incisura cerebelli anterior, or anterior cerebellar notch, which is situated between, the anterior angles of the hemispheres, in front of the vermis cerebelli.
The medullary body (corpus medullare) which is the white center of the cerebellum splits in its median part into two lamina,. a superior, which forms the superior medullary velum and three pairs of connecting bands (peduncles), and an infe rior, which is the inferior medullary velum (Figs. 104 and 106). Separating at an acute angle, the two liminm form the tent of the fourth ventricle.
The inferior medullary velum (velum medullare inferius, Figs. 104 and 122) is the inferior lamina of the medullary body. It is a short plate of white matter, not more than 6 mm. (IA inch) long and is separated from the superior lamina by the angle called the fastigium. It ends in a concave border from which a sheet of epithelium continues down over the fourth ventricle; and together they form the inferior half of the roof of that cavity. Laterally, the inferior velum extends to the flocculus of the hemisphere. Of the worm it covers the nodulus, antero-supe riorly. It bounds, dorsally, the lateral recesses of the fourth ventricle.
The superior lamina of the medullary body joins the cere bellum immediately to the pons. The superior lamina is made up of three pairs of connecting bands (cerebellar peduncles) and the superior medullary velum. It constitutes all the pro longations of the corpus medullare of the cerebellum,. except the inferior velum (Fig. 122).
The brachia conjunctiva (superior peduncles, Figs. 103 and 122) converge as they pass forward and upward to the inferior quadrigeminal colliculi, where they disappear. They are joined to one another by a thin plate of white matter, the rior medullary velum (velum medullare superius). With the velum they form the roof and lateral boundaries of the superior half of the fourth ventricle. They gradually bury themselves in the pons as they proceed upward toward the corpora quadri gemina. Beneath the corpora quadrigemina and the cerebral aqueduct, the brachia conjunctiva cerebelli decussate, and pass into the hypothalamic region of the opposite side. They end chiefly in the red nuclei, which they embrace medially. Near
the corpora quadrigemina each brachium conjunctivum is obliquely crossed by the lateral fillet in its course to the inferior quadrigeminal colliculus.
The superior medullary velum (valve of Vieussens, Figs. 104, io6, and 122), is a trapezoidal sheet of white substance, wider where it fuses with the corpus medullare of the cerebellum than at the mesencephalic end. It forms the floor of the groove be tween the brachia conjunctiva cerebelli and the superior half of the roof of the fourth ventricle. Its lateral borders fuse with and unite the brachia conjunctiva, hence their name. In the median line its posterior surface presents a slight ridge, the frcenulum veli, from either side of which emerges the trochlear nerve.
The corpora restiformia (inferior peduncles of the cerebellum) enter into the cerebellum between the brachium conjunctivum and the brachium pontis (Figs. 103 and 107). They first run obliquely upward and lateralward in the medulla oblongata, where they help to form the floor and lateral boundary of the fourth ventricle; entering the pons, they bend sharply backward into the cerebellum; and, running lateral to the brachium con junctivum, proceed to the cortex.
The brachia pontis (middle peduncles) join the cerebellum to the lateral borders of the pons (Figs. 103, and 122). They are continuous with the transverse fibers in the basilar part of the pons. The brachia pontis in the anterior cerebellar notch are placed lateral to the brachia conjunctiva and the restiform bodies.
Horizontal Sulcus of Cerebellum (Figs. lox, 104 and io6). The cerebellum has one great sulcus which divides it into upper and lower surface. The sulcus horizontalis cerebelli is irregu larly circular in shape; anteriorly its lips are separated by the prolongation of the medullary body from which the sulcus runs backward, dividing the border of each hemisphere and the posterior end of the worm. Rarely the two halves are not con tinuous through the posterior extremity of the worm. In the horizontal sulcus the remaining important sulci of the cere bellum terminate. They are nearly parallel with one another, hence the cerebellum is laminated, not convoluted like the cerebrum. Though the horizontal sulcus is an important land mark in the adult cerebellum, it does not form a primary em bryonic division of the cerebellum but appears late in foetal life (Cunningham).