198 



CHORDATE ANATOMY 



while the ventral portion unites with the third myotome to form the 

 external rectus muscle innervated by the abducent nerve. The dorsal 

 division of the third somite breaks up into loose mesenchyme to form 

 connective tissue. The myotomes of the fourth, fifth, and sixth somites 

 also break up into connective tissue, so that the first persistent trunk 

 myotome is the seventh. In this way, a hiatus occurs in the series of 

 myotomes, and the eye muscles are left as an isolated group which owe 

 their persistence to the fact that they become functionally connected with 

 the eyeball. (Fig. 186) 



If we may draw phylogenetic conclusions from these facts of onto- 

 genesis, we must consider the eye muscles not as relatively young muscles 



LATERAL UNE 



ADDUCTOR OF MANDIBLE' 



Fig. 187. — 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.) 



or as post-otic muscles which have migrated into the pre-otic region, but 

 as the first three myotomes of the vertebrate body. Their present isola- 

 tion may be interpreted as a consequence of the enlargement of the otic 

 capsules. The ontogenesis of cyclostomes and elasmobranchs supports 

 the assumption that in the ancestors of vertebrates, as in Amphioxus 

 today, the myotomes formed an unbroken series extending throughout the 

 entire length of the body. The history of the eye muscles sums up as 

 the transformation of the first three myotomes of an Amphioxus-like 

 ancestor into the six eye muscles of the vertebrates. 



In elasmobranchs the metamerism of the body muscles, which is such 

 a characteristic feature of the musculature of cyclostomes, is retained 

 with slight modification. A more elaborate folding of the myo- 

 tomes of elasmobranchs, however, greatly compUcates 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 



