The fifth cranial nerve (Fig. 138, _V_) (=N. trigeminus=) arises by two roots from the caudal border of the pons, near the lateral end. The ventral root (4) is small; the dorsal one is much larger and soon forms the large semilunar ganglion (_k_) from which three branches (1, 2, and 3) diverge. The ventral root (4) joins one of these branches (1). The =cerebellum= (Fig. 137, _III_) is formed by an increase in size of the cranial portion of the primitive hindbrain. This increase has affected principally the surface of the roof, so that as the cerebellum has grown it has been thrown into many folds, the exact form of which varies in different specimens. The cerebellum has at the same time increased in size and has thus extended laterad as well as caudad and craniad. It thus touches the cerebrum in front (separated from it by the tentorium) and aids it in concealing the midbrain and ’tween-brain in dorsal view, while caudad in the same view it conceals the greater part of the medulla. The connections of the cerebellum with adjacent parts of the brain are also overhung and concealed. The whole surface of the cerebellum is thrown up into numerous folds or =gyri=, separated from one another by deep fissures or =sulci=, which appear at first to render the surface wholly irregular. The entire mass is, however, divisible into a central portion, which from its resemblance to a segmented worm is called the =vermis= (_j_) (its cranial part is the =superior vermis=, and its caudal part the =inferior vermis=), and into lateral portions, the =hemispheres= (_k_). The vermis (_j_) occupies a median longitudinal position, and its gyri and sulci are in the main transverse. It is not directly connected with adjacent parts, and its ventral surface extends farther caudad and craniad than that of the hemispheres. The ventral part of the superior vermis is fitted against the posterior corpora quadrigemina. The =hemispheres= (_k_) may again be subdivided into groups of gyri which have received special names. One of these, the so-called appendicular lobe, fits into the appendicular fossa of the petrous bone. The cerebellum is connected to the adjacent parts by three tracts of fibres, sometimes known as crura cerebelli. The tract connecting it with the medulla oblongata is the =corpus restiforme=; that connecting it with the pons is the =brachium pontis= (Fig. 141, _l_); these have been described. A third tract passes craniad to the corpora quadrigemina (Fig. 141, _p_ and _q_); this is the =brachium conjunctivum= (Fig. 141, _k_).