IX. THE COMPARATIVE ANATOMY OF THE MUSCULAR SYSTEM 



A. GENERAL CONSIDERATIONS 



i. The kinds and origin of muscle. — The muscles of the vertebrate body may be divided 

 into two genera] classes, the involuntary and the voluntary. The involuntary or smooth muscles 

 occur in the walls of the digestive tract and other viscera, and in the skin and certain deriva- 

 tives thereof. They originate through the transformation of mesenchyme cells, which may 

 be of various origins. The majority of the smooth musculature, however, is produced by the 

 mesenchyme of the hypomere, since in development the hypomere closes around the archen- 

 teron and its derivatives (see Fig. 10, p. 43). The voluntary or striated muscles, on the other 

 hand, with certain exceptions specified below, arise from the myotomes. The myotomes, it 

 will be recalled, are those portions of the epimeres remaining after the epimeres have given 

 rise to the sclerotomes and dermatomes. From their original dorsal positions the myotomes 

 grow down between the hypomere and the skin and those of opposite sides meet in the median 

 ventral line. See Figure io-and re-read Section IV of the manual. In this way there is 

 produced a complete coat of voluntary muscles, lying beneath the skin. This muscle coat 

 is divided into dorsal and ventral parts by the horizontal skeletogenous partition which 

 intersects the skin at the lateral line. The muscles dorsal to this septum are called the 

 epaxial muscles, those below the septum, the hypaxial muscles (Figs. i$A, p. 58, 20, p. 62). 



All of the muscles originating from the myotomes are voluntary muscles and are designated 

 as parietal or somatic muscles. Not all of the voluntary muscles are, however, of this kind. 

 In the gill region of vertebrates a system of voluntary muscles is developed for moving the 

 gill arches. Since the gills and related parts are of entodermal origin, the muscles in the 

 walls in the gill region are homologous with the muscles of the rest of the digestive tract 

 and are, in fact, derived from the mesenchyme of the hypomeres. These gill arch muscles are 

 consequently designated as visceral muscles, although, unlike the muscles of the viscera, they 

 are striated and voluntary. There are consequently two kinds of voluntary muscles, iden- 

 tical in structure but different in origin — the parietal or somatic muscles derived from the 

 myotomes and distributed widely over the body and the visceral muscles derived from 

 the hypomeres and found only in the gill region. 



The student should note that the terms muscle and muscular system, when used without 

 further qualification, refer only to the voluntary muscles. In dissecting a vertebrate only the 

 voluntary muscles are studied, as the study of the involuntary muscles properly belongs to 

 histology. It should further be always borne in mind that when the expression visceral muscles 

 is employed this refers not to the involuntary muscles of the viscera but to the voluntary 

 muscles of the gill arches. It is assumed that the student understands the histological differ- 

 ence between smooth and striated muscle. (See K, pp. 25-26.) 



References on the muscular system are: K, pages 129-40; W, pages 189-207, 217-21. 

 245-56; Wd, pages 173-90. 



B. THE MUSCLES OF THE DOGFISH 



i. The parietal or somatic muscles.— Strip off the skin from the dogfish at 

 the base of the tail, in the neighborhood of the pelvic fins. In doing this make 

 a cut through the skin, grasp the cut edge with the fingers, and strip off a piece of 



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