1906.] NERVES OF CliLAMV'DCSELACHUS AXGUfNEUS. 981 



the nerve divides into the pi'e-trematic and post-trematic branches, 

 of which the latter is three times the larger. 



The lyre-tremiatic branch runs along the caudad side of the 

 cartilaginous rays as a single nerve, until an inch beyond the angle 

 of the arch, when it divides. Many minute branches are sent off 

 by the main nerve into the branchial filaments. Near its origin 

 the pre-trematic gives off the visceralis, which passes immediately 

 inwards under the epi-branchial cartilage of the first branchial 

 arch, where it divides into two branches. One of these runs 

 caudad, tlie other cephalad, above the mucosa of the roof of the 

 mouth. 



The post-tre7natic divides into two l^ranches 1 cm. from its 

 origin ; of these the smaller (B) passes along the anterior base of 

 the branchial filaments of the second branchial arch, the posterior 

 (A) and larger branch also runs along the base of the filaments, 

 but more laterad than the smaller branch. Branch A sends side- 

 branches up into the cartilaginous branchial rays and to the 

 muscles thereof, as well as some branches which pass between the 

 rays, apparently to the pre-trematic filaments of the succeeding 

 cleft. Bi-anch A chiefly supplies the muscles and rays, whilst 

 branch B sui^plies filaments. 



(c) Vagus 2 also begins in the ganglionic mass with a flattened 

 ganglion. The nerve then passes obliquely backwards for about 

 4 cm. before it divides into the pre- and post-trematic branches 

 over the third branchial cleft. The pre-trematic is only half the 

 size of the post-trematic. It runs as a main nerve to the end of 

 the arch, and many small branches are sent from it into the 

 filaments. Less than 5 mm. from its origin it gives rise to the 

 small ramus visceralis. This ,bra,nch passes backward and inward 

 over the top of the cleft and under the epi-bi-anchial cartilage, 

 where it suddenly turns forward to be distributed to the mucosa 

 of the dorsal wall of the pharynx. In a second specimen, the 

 visceralis arose directly from Vagus 2. On the other side of the 

 same specimen there was a transverse connecting-branch between 

 the pre-trematic and the visceralis. 



The j)ost-t,re')natiG runs immediately under the mucosa of the 

 cleft at the base of the rays. It divides into two branches about 

 1 cm. from its origin. The larger branch (A) runs along the arch 

 about 5 mm. from and parallel to the smaller branch (B). These 

 two divisions continue their course nearly to the end of the 

 arch, the distribution of the smaller branches being as in Vagus 1. 

 Branch B of Vagus 2 is in proportion smaller than branch B of 

 Vagus 1 . This point should be noticed. 



{d) Vagus 3 can be dissected as a separate nerve, as far into 

 the vagal ganglion as Vagus 1 and 2. Here, however, there is no 

 external trace of the ganglion, but, when the end of the nerve was 

 teased out, a few ganglion -eel Is were found. The nerve passes 

 backwards and somewhat ventrally for 4*5 cm. before it divides 

 into a very small pre-trematic and a large post-trematic. The 

 pre-trematic, which is very small, has the usual course. As with 



