RECURRENT BRANCHES OF ABDUCENS NERVE 375 



embryos. They are commonly small enough to escape notice 

 unless under special scrutiny. In man they are more constant, 

 larger, and apparently of longer duration (found in relatively 

 older embryos) than in any other of these groups. 



Of the twenty-four human embryos in this collection between 

 the first appearance of the abducens nerve and the crown-rump 

 length of 18 mm. only two are entirely lacking in this respect; 

 three others have the recurrent branch on one side only. The 

 two embryos of this group, one of 12 mm. (H. E. C, no. 816), 

 the other of 16 mm. (H. E. C, no, 1128), which lack the recur- 

 rent braneh, show instead long anterior hypoglossal roots running 

 in front of the vagus nerve, between it and the glossopharyngeal. 

 Between 18 mm. and 31 mm. there are again twenty-four em- 

 bryos, of which twelve are provided with the recurrent branch 

 on one or both sides. Beyond this age I have not found it 

 present. As there is evidence that these branches may degen- 

 erate and disappear as early as the 15-mm. stage, the first group 

 of twenty-four embryos would seem to be a fairer index of the 

 usual occurrence of this feature, and one might state therefore 

 that the recurrent branch of the abducens is present in over 90 

 per cent of human embryos; in other words, that their absence 

 is an anomaly, their presence, up to the 18-mm. stage, to be 

 normally expected. 



In spite of this fact, these branches have been noticed only 

 three times, to my knowledge, in papers on the description of 

 individual human embryos. Phisalix, in 1887, writing of the 

 cranial nerves of a human embryo of thirty-two days, 10 mm., 

 seeks to prove that the cranial nerves are similar to the spinal, 

 and after mentioning the dorsal and lateral roots of the vagus 

 and glossopharyngeal nerves continues:^ "Beside these intra- 

 bulbar motor roots I have discovered others which differ in no 

 way from the anterior spinal roots, neither in their origin nor 

 in their connections. Thus for each of these nerves there is a 

 very slender bundle of motor fibers which arises from the bulb, 

 at its base near the median line, and which runs to join the nerve 

 at the distal end of the superior ganglion." No drawing accom- 



• Phisalix, 1887.2, p. 243. 



