196 CHORDATE ANATOMY 



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 

 six eye muscles like those found in all vertebrates up to man. All six 

 are formed from the first three embryonic myotomes. Like the eye 

 muscles of higher vertebrates, they are innervated by the 3rd, 4th, 

 and 6th cranial nerves. Since in cyclostomes the fourth myotome 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. 184, 185, 186) 



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

 stomes. They arise from postbranchial myotomes which send myotomic 



SOP. RECTUS^ 0CUU3M0T0RN. 



N. TBOCHLCARIS^ 

 EXT RECTUS (CUT) 



M. 0BLI0UU5 SUR. 



Cxr. RECTUS — 



NF OBLfQUE 

 INF RECTUS 



Fig. 185. — 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 Car- 

 michael. Courtesy of Houghton Mifflin & Co.) 



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. 184, C) 



The embryos of elasmobranchs 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 

 splitting 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 spUtting occurred along the series of lateral-line sense 

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

 ear vesicle. Each of the two divisions of the first myotome splits again 

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

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

 forms the superior oblique muscle innervated by the trochlear nerve, 



