8o NERVE COMPONENTS OF BONY FISHES. 



Fiirbringer's conclusions from a study of serial sections 

 of Ammocoetes. 



This is a matter of no small importance, for the inner- 

 vation of the ventral musculature, including the pharyngo- 

 clavicular muscles, (cleido-branchialis 5) from the spinal 

 nerves in all vertebrates is the key-stone of Fiirbringer's 

 argument from comparative anatomy for the distinction 

 of paleocranial and neocranial nerves, a distinction of the 

 most fundamental importance for his scheme of the 

 metamerism of the head. According to that scheme, the 

 primordial cranial nerves, back to and including the 

 IX+X+XI complex, in all forms above the cyclostomes 

 belong to those segments which are comprised in the 

 paleocranium of Gegenbaur and from this point upward 

 are to be sharply separated from the spinal nerves, though 

 secondary anastomoses of various forms may occur. 

 Passing up the taxonomic series a progressively larger 

 number of spinal segments become fused with the head 

 and either wholly or partially degenerate. In no case do 

 their nerves fuse intra-cranially with the paleocranial 

 nerves ; on the contrary, they simply atrophy and to their 

 more or less modified vestiges the name " spino- occipital 

 nerves " is given. This group of nerves is further subdi- 

 vided into " occipital nerves," which have lost their spinal 

 character and have become incorporated into the head so 

 that they emerge through foramina in the cranium, and 

 " occipito-spinal nerves," which, though they emerge 

 behind the cranium, yet have suffered some modification, 

 usually the reduction of the dorsal root. The general 

 rule may be laid down, that among the adults of almost 

 every class of vertebrates, the more primitive forms are 

 characterized by more, the higher forms by fewer, of the 

 spino-occipital nerves. The embryology in most cases 

 where it is known recapitulates more or less completely 

 the steps in this reduction. 



In Myxine there are five or six, in Petromyzon two or 

 three spinal nerves which lie cephalad of the first spinal 

 of the lowest selachians (Notidanidae). With the latter 

 forms the paleocranium is completed and the formation of 



