ABERRANT ROOTS AND BRANCHES OF THE ABDU- 

 CENT AND HYPOGLOSSAL NERVES. 



BY 



JOHN LEWIS BREMER, M.D. 



{Demonstrator of Histology, Haraiard Medical School.) 

 With Nine Figures. 



My attention was called to the roots of the abducens by a recent 

 paper by Elze (1907. i) on a 7.0 mm. human embryo, in which 

 he describes the nerve as having two roots, one arising in the usual 

 way from the ventral zone of the medulla, between the origins of 

 the trigeminal and facial nerves; the other, also from the ventral 

 zone of the medulla, but further caudal, about opposite the origin 

 of the glossopharyngeal nerve, and separated from the first root 

 by some branches of the basilar artery. The caudal root runs 

 forward, quite near to the floor of the medulla, and joins the 

 anterior root to make the abducent nerve. This then is merely 

 a caudal prolongation of the origin of the abducens, though show- 

 ing the tendency of these roots to a segmental arrangement, one 

 root between each two branches of the basilar artery, which are 

 also segmental. 



On examining the human embryos in the Harvard Embryo- 

 logical Collection, I find that this is by no means an uncommon 

 occurrence, and may be easily explained by imagining the group 

 of cells in the ventral part of the medulla which gives rise to these 

 fibers to be longer than usual, so that the caudal fibers find it 

 easier to emerge from the brain separately, and run forward out- 

 side the brain to join their fellows, instead of passing through the 

 brain wall. A similar cigin of the abducens is shown in fig. 6, 

 taken from a chick embryo of 25.0 mm.; in the chick this double 

 origin is almost constant. In one of the human embryos and in 

 one of the pig embryos studied, this process was carried so far 

 that there were two distinct abducent nerves on one side, one from 

 the anterior root and one from the posterior or caudal root, having 



