354 AN INTRODUCTION TO ZOOLOGY 



it passes latero-ventrally, and shows from the ventral surface just 

 in front of and to the side of the pons. 



The fifth nerve, the trigeminus, is very large and stout, and 

 comes from the ventro-lateral aspect of the pons below the cerebellum. 

 It arises by two roots close together, an external sensory root and an 

 internal smaller motor root. 



The sixth nerve, the abducens, is quite a small nerve springing 

 from the corpus trapezoideum at the lateral border of the pyramid. 



The seventh nerve, the facialis, is of moderate size, and emerges 

 from the ventro-lateral border of the corpus trapezoideum slightly 

 behind the level of the fifth nerve. 



The eighth nerve, the auditorius, is about the same size as the 

 preceding, and arises just external to it. The main trunk almost 

 at once breaks into two branches, the cochlear and the vestibular, 

 which pass parallel with the seventh to the meatus auditorius 

 internus and are distributed to the correspondingly named part 

 of the labyrinth. 



The ninth, or glosso-pharyngeus nerve, has three roots, and the 

 tenth, or vagus, arises by four roots. These all arise behind one 

 another on the latero- ventral border of the front end of the medulla, 

 slightly medial to the seventh and eighth, and the foremost root 

 comes off just behind the corpus trapezoideum. 



The eleventh, or nervus accessorius, arises by about ten thin 

 roots, and the most posterior of them is well down the spinal cord 

 at about the level of the fifth spinal nerve. The posterior end of it 

 runs forward along the side of the anterior end of the spinal cord, 

 entering the cranium through the foramen magnum. It leaves the 

 cranium through the foramen lacerum posterium in company with 

 the ninth and tenth nerves. 



The twelfth nerve, the hypoglossus, takes origin from eight or 

 nine fine roots from the hinder end of the medulla, nearer the middle 

 line than the ninth or tenth and slightly behind the latter. The 

 foremost roots come from the lateral border of the pyramids, and 

 consequently certain of them are on the same level as some of those 

 of the accessories, but mesial to them. 



Thus it will be seen that although in conformity with long- 

 established custom we still speak of twelve cranial nerves in mammals, 

 there are in reality fourteen, the two extra ones being the nervus 

 terminalis and the nervus vomeronasalis or septalis, as noted 

 previously. 



