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above Jacobson’s anastomosis, then bends sharply down, 
crossing the 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 lying 
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 
