The tenth cranial nerve (X) (N. vagus) arises by about eighteen very delicate rootlets from the surface of the area ovalis. The rootlets are divided into a dorsal and a ventral series. The dorsal series (about twelve) arise in the groove which separates the fasciculus cuneatus medialis from the fasciculus cuneatus lateralis. The ventral series arise somewhat ventrad of this groove. These rootlets are to be distinguished from those of the ninth nerve by their smaller size.
The ninth cranial nerve (IX) (N. glossopharyngeus) arises from the area ovalis from a line craniad of the dorsal line of origin of the vagus roots (X) and between these and those of the auditory (VIII). It arises by a number of rootlets which are larger than those of the vagus (X), with which this nerve is closely associated.
The eighth cranial nerve (VIII) (N. acusticus) appears at the lateral end of the trapezium (l). It arises from an elevation ([Fig. 141], i) which is continued dorsomediad along the cranial border of the area ovalis.
The seventh cranial nerve (VII) (N. facialis) leaves the lateral border of the trapezium (l) near its cranial edge, between the fifth and eighth nerves. It is much smaller than the eighth nerve.
The sixth cranial nerve (VI) (N. abducens) arises by about six bundles from the groove between the pyramids and the trapezii and passes craniad.
2. Metencephalon.
—The metencephalon includes the pons and the cerebellum.
The pons ([Fig. 138], i) is a mass of transverse fibres which forms the ventral and cranial part of the primitive hindbrain. It is a modification of the latter brought about by the development of the cerebellum, and the degree of its development is in direct ratio to that of the cerebellar and cerebral hemispheres. The pons forms a projecting mass of fibres which is marked by a median longitudinal groove, the sulcus basilaris (j), which indicates the course of the basilar artery ([Fig. 121], c). Laterad the fibres of the pons converge somewhat and turning dorsad disappear in the cerebellum, forming the brachia pontis ([Fig. 141], l).
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.