DRIGIN AND STRUCTURE OF THE HEAD 



151 



dorsal roots, is incorporated into the skull (FtiRBRINGER, 

 1897, p. 457). 



Thus among the so-called occipital nerves we must 

 distinguish primarily epibranchial, supplying epibranchial 

 musculature, from secondarily epibranchial or hypoglossus 

 roots, supplying hypobranchial musculature. From the 

 numtier of the latter may be determined the approximate 

 number of post-branchial segments into which the skull 

 reaches. Adding to this number that of the epibranchial 



pl.brach. 



pl.cerv. 



Fig. 32. 



Plexus cervico-brachialis of Heptanchus, 

 after Furbringer's statements, 1897. 



1 primarily epibranchial ventral roots 



2 cervical plexus or hypoglossus. 



3 plexus brachialis. 



segments we get the total length of the skull. This proves 

 to be subject to variation among Selachians, the number of 

 epibranchial (depending on the number of gill-slits) as well as 

 that of post-branchial segments may vary. Thus FuRBRlNGER 

 was wrong in assuming that the skull in all Selachians 

 has equal length and that consequently the last segment 

 and its ventral nerve may always be indicated with the same 

 letter (z). If in Scyllium we call it z, we ought to call it 

 a in Acanthias and 6 or c in Hexanchus and Heptanchus. 

 While z and a in the latter form are prima, ily epibranchial 

 nerves, they have become hypoglossus-roots in forms like 

 Scyllium where the number of gill-slits has decreased. In 



