520 COMPARATIVE ANATOMY. 



the alimentary canal; it ramifies on the pharynx and stomach, and 

 also gives off branches to the heart. Before the vagus trunk gives 

 off the branchial branches, it sends off a large dorsal branch, which 

 passes backwards in the dorsal region ; this is the lateral branch (L), 

 which ramifies in the skin alongside the lateral line of the body, 

 and extends as far as the tail. 



While the nerve-roots, that make up the vagus trunk, leave the 

 myelencephalon together, there are other roots, which belong to 

 the vagus, and which arise from the myelencephalon, underneath those 

 already mentioned; of these there are, at most, five, and, generally, 

 only two or three filaments, each of which reaches the exterior by a 

 special canal in the wall of the cranium. Some pass to the muscles, 

 and some are connected with the first of the spinal nerves ; they 

 may be called the inferior roots of the vagus, while those before 

 mentioned are the superior roots. The foramina of the lower roots 

 lie in the same row as the foramina of the inferior roots 

 of the spinal nerves; the foramen for the complex of superior 

 roots is placed higher up, and is in a line with the foramina of 

 the superior roots of the spinal nerves. 



§ 391. 



From the description that we have just given, it is clear that the 

 whole vagus must be regarded as a complex of a large 

 number of nerves, which are homodynamous with spinal 

 nerves. This is indicated by the various inferior roots, which 

 pass out separately, but much more emphatically by the distri- 

 bution of the trunk, formed by the superior roots. While each 

 branchial branch of the vagus has exactly the same characters 

 as a ventral branch of a spinal nerve; and while, moreover, 

 the branchial arches supplied by it must be regarded as arches, 

 which were primitively part of the cranium (§ 340) ; and while, 

 lastly, each of the other arches (mandibular, hyoid, and first bran- 

 chial) is innervated by a nerve, in just the same way as is a meta- 

 mere of the trunk by a spinal nerve ; the series of the superior roots 

 of the vagus are, on their side, the distinct equivalent of a number 

 of separate nerves, the sum total of which must correspond to the 

 maximum number, at any rate, of the arches supplied by them. 

 This view of the real character of the vagus, advanced by me, is shown 

 to be the correct one by the developmental history of these nerves, 

 which has lately been made out in the Sharks. As there is reason 

 for supposing that, even in the Selachii themselves, there has been a 

 great reduction in the primitive number of the gills, inasmuch as a 

 process of this kind, although, indeed, affecting a small number of 

 gills only, can be observed within the limits of the group, it follows 

 that the extension of the vagus on to a portion of the alimentary tube 

 is explicable as due, not so much to an incursion of the nerve into 

 a territory which was not originally within the area of its distribution, 

 as to the modification of a region, which formerly did carry branchial 



