THE CEREBELLUM.
The cerebellum, or little brain, is a medially situated structure of kidney-like form. It underlies the occipital lobes of the cerebrum, from which it is separated by the large transverse fissure, and lies behind the pons and the corpora quadrigemina and above the medulla oblongata. We distinguish an upper and a lower surface and an anterior and a posterior border. Both surfaces are arched ; the under and more strongly convex surface exhibits in the middle a broad sagittal depression, the vallecula cerebelli, in which lies the medulla oblongata. The anterior border is indented in the mid-line by the incisura cerebelli anterior; likewise, the posterior border by the incisura cerebelli posterior. At the borders of the incisura are the anguli anteriores and posteriores. Front and hind borders meet in the anguli laterales. The median part of the cerebellum, lying between the incisura anterior and posterior, is known as the worm, the vermis cerebelli. The vernzis superior is defined from the lateral portions, the cerebellar hemispheres, by two shallow furrows, while the vermis inferior is more sharply demarcated by deeper grooves. The narrow convolutions, gyri cerebelli, are separated from one another by numerous more or less parallel fissures, the sulci cerebelli, particularly in the worm and the hemi spheres. A deeply penetrating fissure, the sulcus horizontalis cerebelli, extends on each side from the entrance of the pontile arm or middle cerebellar peduncle in the cerebellum along the front border towards the angulus lateralis and thence toward the angulus pos terior. By means of this fissure, each hemisphere is divided into an upper and a lower surface, the facies superior and facies inferior. The sulcus horizontalis is readily located
when we pass from the position at which the pontile arm enters the cerebellum. The sulcus begins lateral to this location, at first penetrating but slightly, and is here dis tinguished by the narrow convolutions of the upper and lower surfaces entering its depth. From the lateral angle, the sulcus proceeds as a deeper cleft along the hind border, more on the lower than the upper surface, toward the incisura cerebelli posterior.
The worm and hemisphere regions of the cerebellum are subdivided into definite lobes by certain more or less deeply cutting fissures. In each hemisphere three lobes are distinguished : lobus superior, lobus posterior and lobus inferior, the individual lobes of the hemisphere always corresponding to definite segments of the worm-region.
A. Lobus Superior. The lobus superior is bounded in front by the incisura cerebelli anterior, at the side by the sulcus horizontalis cerebelli and behind by the sulcus superior posterior. The sulcus superior posterior starts in the sulcus horizontalis somewhat in advance of the lateral angle and passes as a deep curved fissure, directed posteriorly with its convexity toward the hind end of the vermis superior. The sulcus is readily recognized by the different relations of the bounding since those of the superior lobe run obliquely outward and forward, while the lamella of the posterior lobe run parallel.
Passing from before backward, the worm and the hemisphere present the following parts of the lobus superior :