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Show 982 MRS. O. A. MERRITT HAWKES OX THE [Dec. 11, Vagus 2, it sends off a visceral branch which has the same course and distribution as the other visceral branches already described. The post-trematic divides into two parts (A and B) of almost equal size, which run parallel to one another as in Vagus 2. The tine side branches which are so numerous in the equivalent nerve of Vagus 2 cannot be traced here. It is probable that branch A has approximately the same function as the branches which Cole calls " accessory skeletal ' in Chimcera (3. p. 667 <fec.). (e) Vagus 4, 5, 6 and the truncus intestinalis are so closely united that they cannot be separated for a distance of 2 cm. from the vagal ganglion. There is no external trace of separate ganglia, or of a separate compound ganglion for these four nerves. At a distance of 2 cm. from the vagal ganglion, Vagus 4 can be dissected away. This nerve passes obliquely backward to divide just above the fifth branchial cleft into a very fine pre-trernatic and a stout post-trematic. The pre-trematic could be traced only for 15 mm., that is about 1 cm. after it gives off the visceral branch. The post-trematic soon divides into two equal branches which run parallel. There is nothing special about the course of the branches in this segment. The tendency toward a reduction of the pre-trematic is carried further here than in the more cephalic segments. Further, it is noticeable that the pre-trematic of Vagus 4 lies in close association with the two divisions of the post-trematic of Vagus 3. From this association and the reduced size of the pre-trematic branches, it may be deduced that the pre-trematics are being replaced by the post-trematics of the immediately preceding segment. Ultimately such a tendency would lead to the condition found in some Teleosts. As the pre-trematic and post-trematic filaments appear to be equally well developed, the reduction of the pre-trematic nerves can only be accounted for on the supposition that some other nerves are replacing them functionally. (f) Vagus 5 can be dissected from the combined branchial and intestinal nerve about 1*5 cm. beyond the point of separation of Vagus 4. It then passes obliquely downward for about 3 cm. before it divides into the pre- and post-trematics. In one case Vagus 5 divided into the pi e- and post-trematic near the branchio-intestinal riband. The two divisions are of unequal size, the pre-trematic being the smaller in two out of three nerves dissected. The large size of the post-trematic is remarkable, as there are no filaments upon the posterior wall of the sixth cleft, i. e, upon the sixth branchial arch. The pre-trematic passes along the fifth branchial arch immediately under the lining membrane, along the base of the cartilaginous branchial rays. These rays separate this nerve from the two parts of the post-trematic of Vagus 4. Pre-trematic 5 sends small branches outwards among the branchial filaments. On one side, where the forking of Vagus 5 took place at an unusual distance from the cleft, a fine branch united the pre- and post-trematics about 5 mm. from the fork. The visceralis is given off |