ORIGIN AND STRUCTURE OF THE HEAD 97 



has become emancipated from the main stem as the acus- 

 ticus, which is generally considered as a branch ot the 

 facialis. The gill-slit belonging to this segment shows from 

 the beginning differences to the other gill-slits. In Elas- 

 mobianchs it is the spiracle, in Cyclostomes, Amphibians 

 and Teleostei it does not open to the exterior; while 

 in terrestrial Vertebrates the auditory vesicle enters into' 

 close connection with its rudiment which widens to the 

 tympanic cavity in which, in Mammals, ossified parts detached 

 from the first or mandibular arch, the malleus and incus, transfer 

 the vibrations from the tympanic membrane to the labyrinth. 



Backwara extension oj the skull. — A very difficult problem 

 which has given rise to much controversy is that of the 

 backward extension of the skull, in which especially the 

 appreciation of the spino-occipital nerves (FuRBRlNGER) 

 has played an important role in later years. 



According to the views of GeGENBAUR and VAN Wyhe the 

 number of segments incorporated in the vertebral part of the 

 skull is indicated by that of the visceral arches. The last 

 branchial bar, e. g in Scyllium and Pristiurus on which 

 Van Wyhe worked, corresponds to the last segment of the 

 neurocranium and, since this fifth branchial arch lies behind 

 the last gill-slit, there is only one post-branchial segment 

 incorporated into the skull, while the others are epibranchial. 

 According to VAN WVHE (1882) there are in these forms nine 

 head segments, a number which is reduced to seven, the number 

 of the visceral arches, if we do not recognize the praeman- 

 dibular cavity as a somite and reject the idea of a double 

 natuie of the hyoid-segment, for which, as has been recently 

 emphasized by GOODRICH (1S18, p. 5), no serious evidence 

 has ever been found. Of these seven segments there are 

 two pro-otic, four post-otic and epibranchial and one 

 post-otic and post-branchial. 



Ventral occipital nerves. — In Elasmobranchs a number of 

 ventral roots leave the skull in the occipital region, under and 

 behi nd the vagus. Formerly, according to GeGENBAUR's ( 1 87 1 , 

 p. 530) example, they were generally compared to the hypo- 

 gloisus of Amniotes, and GEGENBAUR (1871, p. 521, 1872 

 p. 268) who considered the vagus as a quadrivalent dorsal 

 nerve originally called them the ventral vagus roots. Indeed, 

 as shown first by VAN WYHE (1882, p. 36), their situation 

 in ontogeny corresponds to that of the occipital somites, 

 generally considered, as the region of the vagus, and so do 



