THE CENTRAL NERYOUS SYSTEM. 347 



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 (/). It arises from an elevation 

 (Fig. 141, z) 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 (/) 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. MetcncepJialon. The metencephalon includes the pons 

 and the cerebellum. 



The pons (Fig. 138, 2) 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 develop- 

 ment 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. 0- 



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 () is small; the dorsal one is 

 much larger and soon forms the large semilunar ganglion (k) 

 from which three branches (i, 2, and a) diverge. The ventral 

 root (4) joins one of these branches (i). 



The cerebellum (Fig. 137, ///) is formed by an increase in 

 size of the cranial portion of the primitive hindbrain. This 



