THE ELASMOBRANCH FISHES 243 



oxus and the unsegmented head of the higher vertebrates, and 

 in the latter we may point to the cranial nerves as evidence of 

 a primitive condition of which all traces would, but for their 

 presence, have been lost. 



It should be borne in mind, however, that when we speak of 

 the head as having been segmented, we do not mean that the 

 skull is formed of a number of fused vertebrse, as was once 

 supposed. The segmentation in question is a muscle seg- 

 mentation like that of . Amphioxus, and the head must have 

 been evolved out of the anterior muscle segments before any 

 vertebrae were formed. There is, however, some evidence that 

 a certain number of vertebrae have been fused into the occipital 

 region of the skull. This evidence partly consists in the ex- 

 istence of the occipital nerves, which in the dogfish are repre- 

 sented by three very slender nerves arising from the lower 

 border of the medulla below the roots of the vagus. They 

 pass through fine canals in the occipital region of the skull, and 

 unite immediately outside it to form a slender trunk which is 

 joined by the first five spinal nerves, and increasing in size with 

 each accession, runs back in the anterior cardinal sinus bound up 

 in the same connective tissue sheath with the trunk of the vagus. 

 At about the level of the ductus Cuvieri this trunk separates 

 from the vagus and runs downwards to the anterior border of the 

 pectoral girdle. Here it divides into two branches, one of which 

 passes through a foramen in the pectoral girdle and runs along 

 the ventral surface of the pectoral fin, inosculating with the 

 branches of the brachial plexus ; the other turns forward to 

 course along the anterior border of the coracoid cartilage, and 

 is eventually distributed to the ventral muscles of the throat 

 — viz. the coracomandibularis, the coracohyoideus, and the 

 coracobranchiales. The brachial plexus (fig. 57, Br?) is 

 formed by the union of the sixth to eleventh spinal nerves. 



The sympathetic nervous system of the dogfish is somewhat 

 diffuse, as compared with that of the frog, and the ganglia and 

 ■ nerves connecting them are so minute that they cannot be 

 identified by simple dissection, special treatment with reagents 

 being necessary to bring them into prominence. On opening 

 the posterior cardinal sinus and carefully washing out the 

 blood, one is able to see a number of irregularly-shaped bodies 

 attached to the dorsal wall of the sinus. The most anterior is 

 the largest, and has the form of an elongate lobulated body 



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