On the Cranial Nerves of Elasmobranch Fishes. 533 



The Vagus Complex. The vagus has been long held to represent a 

 large number of nerves which, in most vertebrates, gradually coalesced 

 as the branchial region became reduced in size or otherwise altered. 

 Balfour states that the vagus arises in Elasmobranch s by four gan- 

 glionic roots, while more 'recently Beard and van Wijhe agree in 

 describing the vagus as first appearing in the form of an unsegmented 

 band which later blends with an epiblastic sensory thickening above 

 the four posterior branchial clefts. The nerve for the second true 

 branchial cleft is said, at an early period, to separate from this mass 

 and develop a ganglion. Later the three posterior branchial nerves 

 appear, but for these it is said there is usually only a single ganglion 

 which, however, ventrally "shows a division into three portions." 

 While the anterior portion of the vagus is described as supplying the 

 second branchial cleft, the nerve to the lateral line is described as 

 arising as a secondary formation from the epiblastic sensory thicken- 

 ing above mentioned. The lateral line nerve is usually described as 

 springing from the common trunk, but Balfour, impressed with the 

 importance of this nerve, says it " may very probably be a dorsal 

 sensory branch of the vagus." That this surmise is practically correct 

 will appear from what follows. 



In Lcemargus the vagus complex (12, fig. 1) arises by numerous 

 rootlets disposed in three separate groups, an anterior group including 

 two or three rootlets, a middle consisting of over twenty, and a pos- 

 terior group of five or six rootlets. Hitherto the anterior portion of 

 the vagus has been usually spoken of as Vagus I, or the nerve of the 

 second branchial (first vagus) cleft. In Lcemargus the anterior divi- 

 sion of the vagus appears to be almost entirely concerned in supplying 

 the mucous canal of the lateral line, and hence it may be known as 

 the lateralis nerve, or nerve of the lateral line. Its right to be con- 

 sidered as a special nerve becomes all the more evident when it is 

 mentioned that in some, if not all cases, it presents a ganglionic swell- 

 ing. The lateralis nerve (I, fig. 1) seems in many respects comparable 

 to the ophthalmicus superficialis of the facial complex, and like this 

 latter nerve it springs from the side of the medulla on a higher level 

 than the other divisions of the group to which it belongs. In several 

 cases I have found it arising by one large root and a small accessory 

 rootlet dorsal to and slightly in front of the roots of the glosso- 

 pharyngeal. Having sent a twig from its small rootlet to the glosso- 

 pharyngeal, it extends backwards to enter and traverse with the rest 

 of the vagus the long cranial canal that runs backwards and out- 

 wards behind the auditory capsule. Soon after entering the canal ic 

 seems to blend with the rest of the vagus, but with care the whole or 

 at least most of the fibres springing from above the glossopharyiigeal 

 can be shown to be directly continuous with the nerve of the lateral 

 line. Soon after entering the canal it gives off a slender branch which, 



