Motor System — Muscles and Skeleton 



93 



Fig. 92. Diagrams showing budding of hypoglossal- and pectoral or fin muscles 

 from trunk myotomes in an elasmobranch embryo. (A) Lateral view after Braus. 

 (2-6) Visceral arches. (B) Cross section in region of pectoral fin-fold. (Courtesy, 

 Neal and Rand: "Chordate Anatomy," Philadelphia, The Blakiston Company.) 



toral and pelvic fins are developed from mesoderm which grows out 

 from several myotomes adjacent to the base of the "limb-bud" (Fig. 

 92). Presumably the appendicular muscles of primitive vertebrates 

 were derived from segmented mesoderm. There is some doubt, how- 

 ever, as to the exact source of the appendicular mesoderm in the 

 tetrapods. It is possible that it may be derived in part from the unseg- 

 mented ventrolateral mesoderm of the embryo — i.e., from the somato- 

 pleure which otherwise becomes the parietal peritoneum (Fig. 90, p). 

 The muscles of a lateral fin of a fish are few and simple (Fig. 93) . 

 These fins are ordinarily used not for propulsion but for steering and 

 balancing. While the skeletal rays which stiffen the fin may be jointed, 

 the protruding part of the fin works as a whole in relation to its basal 

 skeleton or girdle, which is embedded in the muscle of the body-wall. 

 A muscle extending from the dorsal part of the girdle onto the dorsal 

 surface of the fin serves as an elevator (or extensor) of the fin, its action 



LATERAL UNE 



TRAPEZIUS M 



CONSTRICTORS OF GILLS 

 SPIRACLE 



ADDUCTOR OF MANDIBLE 



Fig. 93. The superficial muscles in the shoulder region of a dogfish, Squalus. 

 (After A. Brazier Howell. Courtesy, Neal and Rand: "Chordate Anatomy," 

 Philadelphia, The Blakiston Company.) 



