THE ROOTS OF THE VAGUS NERVE. 427 



the last branch of the vagus to the first spinal nerve, during 

 which it is not connected with the root of any nerve ; vide 

 fig. 15 b, com. This space probably contained originally the 

 now lost branches of the vagus. In many transverse sections 

 where the dorsal commissure might certainly be expected to 

 be present it cannot be seen, but this is perhaps due to its 

 easily falling out of the sections. I have not been able to prove 

 that the commissure is continued forwards into the auditory nerve. 



The relation of the branches of the vagus and glossopharyn- 

 geal to the branchial clefts requires no special remark. It is 

 fundamentally the same in the embryo as in the adult. The 

 branches at the posterior side of the clefts are the first to appear, 

 those at the anterior side of the clefts being formed subsequently 

 to stage K. 



One of the most interesting points with reference to the 

 vagus is the number of separate strands from the brain which 

 unite to form it. The questions connected with these have been 

 worked out in a masterly manner, both from an anatomical and 

 a theoretical standpoint, by Professor Gegenbaur 1 . It has not 

 been possible for me to determine the exact number of these in 

 my embryos, nor have I been able to shew whether they are as 

 numerous at the earliest appearance of the vagus as at a later 

 embryonic period. The strands are connected (PL 17, fig. 5) 

 with separate ganglionic centres in the brain, though in several 

 instances more than one strand is connected with a single 

 centre. In an embryo between stage O and P more than a 

 dozen strands are present. In an adult Scyllium I counted 

 twelve separate strands, but their number has been shewn by 

 Gegenbaur to be very variable. It is possible that they are 

 remnants of the roots of the numerous primary branches of the 

 vagus which have now vanished ; and this perhaps is the ex- 

 planation of their variability, since in the case of all organs 

 which are on the way to disappear variability is a precursor of. 

 disappearance. 



A second interesting point is the presence of the two connect- 

 ing commissures spoken of above. It was not till comparatively 

 late in my investigations that I detected the dorsal one. This 

 has clearly the same characters as the dorsal commissure already 



1 Loc. cit. 



282 



