133 



above Jacobson's anastomosis, then bends sharply down, 

 crossing tbe latter externally, and passes backwards and 

 downwards until it reaches the first branchial arch, where 

 it divides into two almost equal branches. One now 

 passes straight downwards on to the anterior or concave 

 aspect of the arch. This is the uppermost and smaller of 

 the two, and may be called, like the similar divisions of 

 the RR. post-trematici of the vagus, the R. post-trematicus 

 dorsalis (text-fig. 4). It courses forwards in this position 

 giving off branches until it became too inconspicuous to 

 be followed, which happened before the arch joined the 

 first and second basibranchials. The other division (R. 

 post-trematicus ventralis), the lower and larger of the two, 

 after continuing backwards for a bit, bent downwards and 

 forwards to reach the posterior or convex aspect of the 

 arch, curving externally round the elbow formed by the 

 junction of the epi- and cerato-branchials. It then follows 

 the arch forwards in the same position, gives off a branch 

 above, and ultimately reaches the junction of the first 

 branchial arch with the basi-branchials. Thereafter it 

 arrives at the lateral edge of the branchial isthmus, cross- 

 ing forwards under the hypobranchial. In front of the 

 latter, it turns sharply upwards, and reaches the dorsal 

 surface of the isthmus near the lateral edge, and Ij'ing 

 just under the mucous membrane at the side of and above 

 the first basi-branchial. Just over the cerato-hyal it 

 anastomoses with the first branchial division of the vagus. 

 It is now on the tongue, and tapers down and is lost under 

 the mucous membrane of its dorsal surface, thus reaching 

 much further forwards than the dorsal division. 



We now proceed to describe the vagus complex, and 

 we find that this is formed by the Ramus lateralis vagi, 

 belonging to the lateral line system, and having only a 



