RECURRENT BRANCHES OF ABDUCENS NERVE 381 



The earliest abducens root fibers in man leave the medulla 

 wall at right angles to its surface, and continue growing in this 

 direction for a considerable distance. In an embryo of 7.5 

 mm. (H. E. C, no. 256) and in another of 9.6 mm. (H. E. C, 

 no. 1001) the fibers point straight away from the brain wall. 

 In one of 6 mm. (H. E. C, no. 2094) the very few fibers as yet 

 present curve shghtly at their distal ends, and point some crani- 

 aUy, some caudally, in about equal numbers. In an embryo 

 of 4.6 mm. (H. E. C, no. 374) a single rootlet divides, sending 

 a recurrent branch caudally and the main nerve cranially, both 

 short, but of about equal size and length. Another rootlet is 

 just emerging from the brain on one side a short distance crani- 

 ally. Incidentally it will be noted that the total length of these 

 embryos is not a reliable measure of their development at this 

 time, the smallest of this group being really the oldest in general 

 configuration as well as in the growth of the abducens nerve. 

 The first rootlet, then, seems to be attracted both forward and 

 backward with equal force, and the individual fibers seem to 

 hesitate, and often interdigitate in their final course. I have 

 already noted that Dohrn found these rootlets in a similar con- 

 dition in Torpedo, but after a short caudal growth they usually 

 all turned forward. In pig, sheep, and rabbit, on the other hand, 

 the first abducens roots turn cranially almost from the start. 

 Apparently, then, there is in man some force acting on the earliest 

 growing fibers to turn some of them caudally, a force which is 

 lacking or of insufficient strength at the crucial moment in 

 other mammals. 



Another point to be taken into consideration is the relative 

 time of the emergence of the numerous abducens rootlets. Belo- 

 golowyi" makes the observation that in chicks the anterior spinal 

 nerves appear earliest, and the motor nerves of the brain later, 

 but that the order of development of this forward group is subject 

 to great variation. Of the abducens roots particularly he makes 

 this generalization, 11 that in birds they develop one after the 

 other from behind forward, while in mammals the reverse is 



»» Belogolowy, 1910, p. 270. 

 " Belogolowy, 1910, p. 322. 



