268 



COMPARATIVE ANATOMY 



not the trochlear, which is the nerve of the second myotome, but the 

 abducens, the sixth nerve. We appear to have here to do with a case of 

 nerve piracy, in which a nerve of one myotome has invaded the territory 

 and usurped the place of another nerve. Similar cases of nerve substitu- 

 tion are not unknown, and the innervation of a myotome by nerves of 

 more than one segment is the rule rather than the exception. A difhculty 

 which is perhaps more serious is that the abducens nerve appears not to 

 be the original nerve of the third myotome, but to be a post-otic nerve 

 which has invaded pre-otic territory. The causes of such a nerve sub- 

 stitution are as obscure as those of the trochlearis chiasma (Fig. 505). 



Muscles in Elasmobranchs. The metamerism which is such a char- 

 acteristic feature of the musculature of cyclostomes is retained with slight 



LATERAL LINE 



CONSTRICTORS OF GILLS 

 SPIRACLEv 



ADDUCTOR OF MANDIBLE' 



Fig. 222. — The superficial muscles in the shoulder region of Squalus. From such 

 relatively simple beginnings have evolved the complex muscles of the arm and shoulder 

 of man. The flexor protractor muscle which corresponds to the deltoid muscle in 

 mammals is covered in the figure by the posterior gill constrictor. (Redrawn after 

 A. Brazier Howell.) 



modification in elasmobranchs. A more elaborate folding of the myo- 

 tomes of elasmobranchs, however, greatly complicates their form. The 

 cause of this folding is unknown. The total amount of muscle remains 

 the same; and although the myocommata are folded along with the 

 myotomes, so that the surface of attachment of the muscles is increased, 

 it has not been proved that this increase is adaptive. 



A novelty first appearing in this group, is the division of the lateral 

 trunk myotomes by a horizontal connective tissue septum into epaxial 

 and hypaxial groups of muscles. Five post-branchial myotomes send 

 buds anteriorly into the floor of the pharynx to form the hypobranchial 

 musculature innervated by the hypoglossal nerve. 



The most important advance, however, made by the elasmobranchs 

 is the first appearance in vertebrates of the muscles of pectoral and pelvic 

 fins. As the myotomes extend ventrally in the body-wall, hollow epithe- 

 lial buds branch off laterally into the fin anlagen. The appendicular 



