THE MUSCULAR SYSTEM 265 



motor nerve. The dorsal of the two moieties of the second myotome 

 forms the superior obUque muscle innervated by the trochlearis nerve, 

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

 external rectus muscle innervated by the abducens nerve. The myotomes 

 of the fourth, fifth, and sixth somites break up into connective tissue, 

 so that the first persistent trunk myotome is the seventh. In this way, 

 a hiatus is left 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. 220*) 



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

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

 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. 



Are the three myotomes which form the eye muscles exactly homo- 

 logous with the first three myotomes of amphioxus? On account of the 

 absence of eyes, ears, and brain vesicles in amphioxus, it is difficult to 

 answer this question categorically. It is not even agreed that the mouth 

 of amphioxus is the homologue of the vertebrate mouth. 



That the first myotome of amphioxus may be compared with the first 

 myotome of elasmobranchs is supported by the evidence that these 

 myotomes have the same relation to the anterior endodermic diverticula 

 of amphioxus that they have to the anterior head cavities of ganoids and 

 elasmobranchs. In ganoids the anterior head cavities, hke the anterior 

 endodermic diverticula of amphioxus, are paired outpocketings of the 

 anterior blind end of the enteric cavity and only secondarily acquire 

 openings to the exterior, as also do the proboscis cavities of the hemi- 

 chorda and the left cavity of amphioxus. Since, therefore, these cavities 

 are the most anterior in the chordate body and share the peculiarity of 

 opening by pores to the exterior, and since they have similar topographic 

 relations to the first permanent myotomes and to the mouth, the assump- 

 tion of their exact homology seems justified. 



If, then, we make this assumption, 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. Primarily, as in 

 amphioxus, the three anterior myotomes were members of an unbroken 

 series of segmented muscles extending throughout the entire length of 

 the body. When lateral line organs and enlarged cranial ganglia asso- 

 ciated with them evolved, the anterior myotomes became split lengthwise 



