ORIGIN AND STRUCTUR E OF THE HEAD 107 



be applied to the hypobranchial muscles, though from the 

 observations at hand we cannot yet conclude that there is 

 a complete correspondance. Thus we reach the conclusion 

 that in Selachians the number of occipital nerves must 

 be approximately indicative of the number 

 of post-branchial segments incorporated 

 into the skull; approximately, since we have seen 

 that the last epi-branchial myotome also may perhaps con- 

 tribute to the hypobranchial musculature. Moreover we may 

 take into account only those occipital nerves that join the 

 cervical plexus and thus pass behind the gill-slits to the 

 hypobranchial muscles, for this is not the case with all. 



Not all occipifil nerves hypoglossus roots.— In some 

 Elasmobranchs, especially in those forms where the number 

 of occipital nerves is relatively great {Hexanchus, Heptan- 

 chus), the anterior one or two occipital nerves do not 

 participate at the formation of the plexus but remain inde- 

 pendent and directly run to the epibranchial muscles that 

 are supplied by them (cf. fig. 32). This seems to me a circum- 

 stance of special interest. FuRBRINGER (1. c. p. 394) con- 

 siders it as a secondary emancipation from the plexus. Since, 

 however, it is just in primitive forms, as Hexanchus and 

 Heptanchus where the nerves designated as v and w are 

 independent, that this phenomenon is met with, it appears 

 much more probable that we have to do here with a pri- 

 mitive feature and that these neives v and w are to be 

 compared with the ventral roots of the anterior, 

 primarily epibranchial, myotomes of Petromyzon, 

 being in the latter the anterior six post-otic ones. These 

 nerves also supply exclusively epibranchial musculature. 



The maximum number of occipital nerves joining the 

 cervico-brachial plexus in Selachians is only three, being 

 X. y, z, according to FuRBRINGER. Their branches partly leave 

 the plexus again to supply epibranchial musculature, and they 

 partly innervate the hypobranchial muscles. By carefully 

 splitting the plexus into its components FuRBRINGER could 

 demonstrate that it is even subject to serious doubt whether 

 the nerve jc ever partakes in the innervation of the hypo- 

 branchial musculature. In a table given by him (i.c.p 404) 

 of Ihe occipital and spinal nerves contributing to the inner- 

 vation of the hypobranchial muscles in different Selachians 

 we find .x. with a query, mentioned only for Hexanchus and 

 Heptanchus, while at another place in this table only y and z 



