376 JOHN LEWIS BREMER 



panies this description, but it is evident that either anterior 

 hypoglossal or posterior abducens roots are indicated. Thyng^ 

 speaks of the presence in a 17.8-mm. embryo of a caudal aberrant 

 root on each side running from the region of the vagus and glosso- 

 pharyngeal nerves to join the abducens. This embryo is in this 

 laboratory, and represents one of the better examples of recur- 

 rent branches — the root to the abducens being actually a branch 

 from this nerve, ending below the upper gangUon of the vagus. 

 Barniville^ describes a posterior root or branch of the abducens 

 in a human embryo of 8.5 mm., which, joining a branch from the 

 most anterior hypoglossal nerve, turns dorsally, like a dorsal 

 ramus of one of the spinal nerves, and extends to the level of 

 the ganglionic commissure of the vagus, where it comes very 

 close to the brain wall. Barniville is in doubt whether it actually 

 has a connection here or not, and therefore in doubt whether to 

 call the fibers sensory roots of the hypoglossal and abducens, 

 or dorsal rami of these nerves. He refers to my earlier figures, 

 where similar dorsal rami are shown. Streeter, in his article 

 on the cranial nerves in the Keibel-Mall Textbook of Embryology, 

 mentions the aberrant roots of the abducens and hypoglossal 

 nerves as possible results of a caudal extension of the abducens 

 nucleus or cephalic extension of the hypoglossal nucleus, but 

 gives no hint of their frequency, and does not mention or figure 

 the recurrent branches. Elze, in 1907, describes two roots of 

 the abducens in an embryo of 7 mm., one arising far caudally; 

 but this is not a recurrent branch, as the posterior fibers pass 

 forward from the brain to join the anterior root on the way to 

 the eye muscles. 



Several reasons may be considered for the failure of other 

 investigators to notice these recurrent branches of the abducens 

 in man: the usually transverse plane of section places the abdu- 

 cens roots at a disadvantage, cutting them across where they 

 are smallest at their origin from the medulla floor and where 

 small branches of the vessels entering the medulla are very 

 numerous and in close relationship with the nerve roots, thus 



^ Thyng, 1914, p. 56. 

 8 Barniville, 1914. p. 11. 



