388 JOHN LEWIS BREMER 



or of the branchial muscles, and, because of the probable late 

 development of the eye and its accompanying structures, were 

 relieved from the usual counter-attraction of the eye muscles. 

 Even the most anterior rootlets thus turned caudally, not, to be 

 sure, to the occipital muscles, but to the ventral condensation 

 for the muscles supplied by the facial nerve. A little later, 

 after the abducens fibers had all emerged and taken a caudal 

 course, the eye muscles developed and attracted the oculomotor 

 nerve, which, since the external rectus muscle was unprovided 

 by its usual nerve, sent an unusual branch to this muscle also. 

 Of the recurrent branches of the abducens we can imagine that 

 only the most anterior on one side has persisted, all the others 

 undergoing the degeneration commonly found before this age. 

 Probably the one remaining would soon have disappeared; in 

 fact, its caliber at its origin is already much less than at its union 

 with the facial nerve, and in the brain the nucleus of the abducens, 

 apparently entirely absent on the opposite side, is very doubtfully 

 represented by a slight grouping of cells. 



It seems probable, then, that the absence of the abducens 

 nerve in a human subject is due to some very unusual retardation 

 of the development of the eye, which causes the abducens fibers, 

 released at the crucial moment from any anterior attraction, to 

 turn caudally, thus leaving the external rectus muscle free to be 

 annexed by some other nerve. With the degeneration of the 

 recurrent abducens fibers, as is always to be expected, the nerve 

 in these cases is completely lost, and no trace of its original 

 emergence from the brain is carried into adult life. 



SUMMARY 



Recurrent branches of the abducens nerve consist of fibers 

 whose growth is in a caudal or ventrocaudal or dorsocaudal 

 direction. They are present infrequently in Acanthias, Heptan- 

 chus, and pig, absent in chick, lizard, sheep, and rabbit. They 

 are most frequent in man, where they appear in about 90 per 

 cent of embryos up to 18 mm., and less frequently up to 31 mm., 

 since they msiy begin to degenerate at 15 mm. Degeneration 



