Motor System — Muscles and Skeleton 



I. Muscles 



General Body-Muscles 



The term "body-muscle" designates the muscles which effect lo- 

 comotion and other major movements of the body or regions of it, in 

 distinction from visceral muscles such as occur in the walls of the 

 digestive tube and other internal organs. Body-muscle and visceral 

 muscle are histologically different (see p. 96). 



In an adult fish the body-muscle is segmented — that is, it is divided 

 transversely into blocks or plates which constitute an anteroposterior 

 series of similar muscular units, the myomeres. The muscle-fibers of 

 the myomere extend anteroposteriorly (Figs. 88, 89). Between adja- 

 cent myomeres is a thin layer of tough fibrous connective tissue, a 

 myoseptum or myocomma, which binds the myomeres together and 

 transmits the pull from one to the next. The myomere is usually bent 

 into highly complex form, so that the edges of the myoseptums, as they 

 appear at the external surface of the body-muscle, follow sharply zig- 

 zag lines (Fig. 89). In well-cooked fish the myoseptums are so softened 

 that the meat readily separates into thin flakes, the myomeres. In a fish 

 the number of segments may run into hundreds. On each side of the 

 body a myomere is horizontally divided into a dorsal (epaxial) part 

 and a ventral (hypaxial) part, the two being separated by a hori- 

 zontal septum of connective tissue. In fishes the horizontal septum is 

 approximately halfway between middorsal and midventral lines 

 (Fig. 88C). 



In early embryonic stages of all vertebrates, the muscle-form- 

 ing mesoderm gives rise to pairs of masses lying close upon either side 

 of the median dorsal nerve-tube and notochord and forming a series 

 extending from head to tip of tail (Figs. 5, 90). In a fish these meso- 

 dermal somites, or myotomes, develop directly into the myomeres 



89 



