1895.] Dr Gashell, The Origin of Vertebrates. 23 



The history of the Vertebrate muscular system. 



It would take too long to trace out the changes which have 

 taken place in the arrangement of the voluntary muscles as we 

 ascend from the fish to man ; it is sufficient to say that such a 

 comparative myology has been largely worked out, with the result 

 that the voluntary muscular system of Vertebrates falls into two 

 well-marked groups, viz. the body or somatic muscles and the 

 muscles connected with the branchiae and viscera. This division 

 is so marked and so characteristic in the cranial region that the 

 segmentation of the body is looked upon as double, viz. Branchio- 

 merie and Mesomerie, and the researches of v. Wijhe have shown 

 that in the head region of Elasmobranchs segmental sets of 

 well-defined ventral and dorsal plates of mesoblast give rise 

 to the muscles of the splanchnic or branchial segments and to 

 those of the somatic segments respectively. Passing to the 

 Ammoccetes we see these two sets of muscles still more separated 

 from each other, so that we may conclude that such double seg- 

 mentation, being more and more conspicuous as we descend, is a 

 marked characteristic of the immediate ancestor of the Vertebrate. 



The importance of this division of the voluntary muscles of the 

 body into a splanchnic and somatic group is still further increased 

 by the fact that the nerve supply for these two sets of muscles is 

 also well defined and separate in the cranial region. Thus we see 

 that the splanchnic voluntary muscles are innervated by a seg- 

 mental set of nerves having well-defined cell origins in the central 

 nervous system, apart from the groups of cells giving origin to the 

 somatic muscles. These nerves are V m , VII, IX, X. 



Thus we come to the conclusion that the double segmentation 

 of the body was brought about because the immediate ancestor of 

 the Vertebrates possessed a double muscular segmentation of which 

 the ventral segments were connected with the movement of a 

 branchial and allied apparatus, and that the cells of origin of 

 this more perfectly segmented part of the body were grouped in 

 well-defined groups, separate from the cells of origin of the less 

 perfectly segmented somatic muscles. 



Again, we see that the conditions which exist among the Crus- 

 taceans and Arthropods, as described by Hardy, exactly fulfil the 

 requirements just mentioned, for he has shown that the well- 

 marked segmented muscles of the appendages, whether branchise 

 bearing or not, are innervated by nerves arising from well-defined 

 cell groups in the ganglia, quite distinct from the more diffused 

 cell groups which give origin to the less completely segmented 

 somatic muscles. 



We see, in fact, that just as the consideration of the skeletal 

 tissues led directly to the conclusion that the branchial bars were 



