64 F. W. THYNG 



consequence of its variation in other human embryos. Streeter 

 ('04) places the level at the third or fourth cervical, but finds 

 that sometimes it is more caudally placed. 



The main trunk of the nerve arches cephalad and ventrally 

 under the hypoglossal ganglia (Gg.hyp.) to become incorporated 

 with the vagus ventral to the jugular ganglion. .At the cephalo- 

 ventral border of the ganglion nodosum of the vagus, the ramus 

 externus (R.ex., plate 3) of the accessory nerve curves laterally 

 and dor sally around the lateral side of the internal jugular vein 

 (plate 6) to the developirg sterno-cleido-m.astoid m.uscle. 

 From here it passes around the cephalic part of the adjacent 

 lymph sac to reach the anlage of the trapezius muscle. In the 

 angle between the internal jugular vein and the lymph sac 

 (S.jug.) it communicates with the great auricular nerve. 



N. hypoglossus. Numerous, small, hypoglossal rootlets issue 

 from the ventral wall of the myelencephalon on either side, and 

 each group converges to form four trunks. These, after crossing 

 the lateral side of the vertebral artery (A.vert.), further join to 

 form the two main roots of the hypoglossal nerve {N.hyp., 

 plates 2 and 3). 



On the right side, in addition to the four trunks seen on the 

 left, a vestigial root joins the fourth trunk (plate 3). Such ves- 

 tigial roots of the hypoglossal nerve have been found by Bremer 

 ('08) to occur frequently in embryos of man, pig, sheep and dog, 

 and almost constantly in the turtle and chick. 



It will be noted that the caudal of the four hypoglossal roots 

 on the left (plate 2) passes through an arterial fenestra of the 

 vertebral artery. A similar, but more extensive anastomosis of 

 arterial branches with the vertebral, enclosing the hypoglossal 

 roots has been shown by Elze ('07), (Taf. 15, fig. 2). They show 

 how, by means of island-formation, the vertebral artery in the 

 adult may come to pass between the roots of the hypoglossal 

 nerve or, in rare cases, extend lateral to the entire nerve. The 

 hypoglossal ganglia have been considered with the description of 

 the vagus. 



The hypoglossus receives a branch from the first cervical nerve 

 (one from the second cervical in addition on the right), and ex- 



