386 JOHN LEWIS BREMER 



which turn doi sally, along the wall of the hind brain. Here 

 we must suppose, if the musculature plays any part in determin- 

 ing their course, that certain cells of the somites migrate dorsally, 

 as in the body somites, attracting the nerve fibers with them 

 as dorsal rami. The same possibility is invoked by Neal and 

 others to explain the course of the trochlear nerve, and it may 

 be of interest to record that in one embryo of 16 mm. (H. E. C, 

 no. 2095) the trochlear itself, though otherwise normal, has 

 dorsal rami in the form of short branches which pass dorsally 

 from each nerve just lateral to the chiasma. The lateral muscles 

 have retained their position, the ventral or branchial and the 

 dorsal muscles have migrated, carrying their nerve branches 

 with them. 



In connection with this discussion of the probable causes of 

 the development of the recurrent branches of the abducens, 

 it is of interest to describe a human embryo of 18 mm. which 

 shows a total absence of this nerve to the external rectus muscle. 

 The embryo, which was cut in the sagittal plane, was found too 

 poorly preserved for the regular Embryological Collection, and 

 so kept in a supplementary group, and numbered 393. In spite 

 of the poor preservation, however, all nerves can be readily 

 followed, and such structures as muscles and organs easily 

 recognized ; the brain and allied structures have, as usual, suffered 

 the most damage. 



Complete absence of the abducens nerve on one or both sides 

 is recorded as a rare anomaly in text-books of anatomy, but I 

 know of no mention of its occurrence in embryonic stages. The 

 external rectus muscle is in these cases usually supplied by a 

 branch from the oculomotor nerve, and that is true in this 

 instance. No other anomalies, except those noted in connection 

 with this nerve, were found on casual examination. 



On the left side the abducens is entirely absent; no trace of 

 fibers arising from the medulla in the usual ventral position can 

 be found between the roots of the oculomotor and those of the 

 hypoglossal nerve. A peculiarity in the lateral nerve roots is 

 the presence of a fairly large bundle which emerges between 

 and in line with the lateral roots of the trigeminal and facial 



