204 THE BRANCHES OF THE VAGUS NERVE. 



merited. It deserves to be noticed tliat the dorsal commissure 

 has a long stretch, from the last branch of the vagus to the 

 first spinal nerve, during which it is not c(>nnected with the 

 root of any nerve; vide fig. 15 6, 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 for- 

 wards 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 wdth 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\ 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 (PI. xvi. fig. 5) 

 with separate ganglionic centres in the brain, though in several 

 instances more than one strand is connected wdth 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 

 connecting commissures spoken of above. It was not till com- 



' Loc. cit. 



