NERVOUS SYSTEM OF A TWO-HEADED PIG EMBRYO 407 



The eighth nerve. The vestibular and cochlear contributions 

 to the conspicuous ganghon acusticum are readily identified. 

 Their roots enter the brain in small root bundles composed of 

 several fascicles and more or less distinguishable one from the 

 other. The common entrance is at the lower border of the 

 medulla, caudolateral to the facial, and overhung by the lateral 

 recess of the fourth ventricle (figs. 4, 12). 



IX, X, and XI. Glossopharyngeus, vagus, and accessorius. 

 These three nerves are also grouped together because of the 

 intimate relationship that exists at their superficial origins. 

 The accessorius arises by a series of small bimdles in the region 

 of the cervical spinal nerves, and mingles with similar rootlets 

 of the vagus as the latter emerges along the outer surface of the 

 medulla (figs. 4, 13 to 17). Beyond the ganghon nodosum of the 

 vagus I was not able to trace the independent course of the 

 accessorius continuously, but some of its peripheral fibers were 

 identifiable among the muscle masses of the shoulder. 



The glossopharyngeus is recognized at its superficial origin as 

 a large bundle of rootlets at the rostral end of the vagus series. 



The peripheral distribution of the ninth and tenth nerves is 

 easily followed (figs. 4, 7). The vagus courses along the neck 

 in typical normal manner, giving ofT a well-defined auricular 

 branch which receives an anastomosis from the seventh, a 

 recurrent laryngeal, and cardiac branches. The ganghon no- 

 dosum is very large. 



The ninth nerve runs parallel to the tenth, at first medial, 

 then rostral, then medial again, and is lost in the region of the 

 larynx and common root of the tongues. 



XII. Nervus hypoglossus. The twelfth nerve arises by several 

 rootlets in linear series with the ventral roots of the cervical 

 spinal nerves, and therefore medial to the 9-10-11 series (figs. 

 4, 16, 17, 18). Soon after piercing the cranium these rootlets 

 unite in a common trunk which twists in a. semispiral around 

 the ganghon nodosum to its outer border, and at its caudal 

 extremity turns abruptly rostralward to its termination in the 

 tongue of head B. 



