264 COMPARATIVE ANATOMY 



the gill cartilages, and are differentiated into levators, depressors, and 

 constrictors of the gills. 



The lateral trunk muscles of cyclostomes strikingly resemble those of 

 amphioxus. In the region of the body cavity, on the ventral side, an 

 external layer of oblique muscles is differentiated. The most important 

 evolutionary advance, however, appears in the differentiation of six 

 eye muscles. Paired eyes first appear in this group, and with them the 

 same six eye muscles as in all vertebrates up to man. All six are formed 

 from the first three embryonic myotomes. It is generally assumed that, 

 like the eye muscles of higher vertebrates, they are innervated by the 

 3rd, 4th, and 6th cranial nerves. Since in cyclostomes the fourth myo- 

 tome of the embryo forms the first permanent trunk myotome, all the 

 myotomes of the embryo persist in the adult. Of none of the higher 

 vertebrates is this true. (Figs. 218, 219, 220.) 



Sup. rectus^ ^^_ oculomotor k 



Tf«OCMLEABIS^_ 

 M. OSLIQUUS SUR. 



EXT. RECTUS— — 



Fig. 219. — Diagrams of the eye muscles of man. A shows the left eye-ball and 

 associated muscles viewed from the outer side. B is the left eye-ball with associated 

 muscles and nerves viewed from the median side. (Redrawn after "Warren and 

 Carmichael, Coiirtesy of Houghton Mifflin & Co.) 



Hypobranchial muscles, lacking in amphioxus, first appear in cyclo- 

 stomes. They arise from postbranchial myotomes which send myotomic 

 buds ventrally and anteriorly below the gills as far forward as the mouth. 

 The development and nerve relations of this hypobranchial musculature 

 prove that it is the homologue of the tongue and throat muscles, which 

 in higher vertebrates, are innervated by the twelfth nerve, the hypoglossal. 

 Cyclostomes, however, have no true tongue. The hypobranchial muscles 

 function as a part of the lateral trunk muscles. (Fig. 218.) 



A further evolution of muscles also appears in elasmobranchs. The 

 embryos of this group provide a clue to the history of the eye muscles, by 

 demonstrating that the differentiation of the three anterior myotomes into 

 the six eye muscles involves primarily a longitudinal sphtting of the 

 myotomes into dorsal and ventral moieties such as happens also in the 

 first and second post-otic myotomes of cyclostomes. The facts suggest 

 that the splitting occurred along the series of lateral line sense organs, 

 which at one time may have included the lens of the eye and the ear 

 capsule. Each of the two divisions of the first myotome spHts again 

 lengthwise, thus making the four eye muscles innervated by the oculo- 



