14 



The Muscular System 



INTRODUCTION 



There are several kinds of muscles in the vertebrate body. 

 These can be described as being of two basic morphological 

 types: smooth and striated. The transition between these is 

 observed in the striated nature of the ciliary muscles of the 

 bird. In most vertebrates these muscles are smooth. The 

 striated muscles are of two kinds: the cardiac muscle of 

 the heart and the skeletal muscles of the body. Skeletal 

 muscle is derived from three embryological sources. In the 

 head region of fishes there is a combination of ectodermally 

 derived neural crest tissue and mesenchyme, which gives 

 rise to the skeleton of the branchial arches and to its intrinsic 

 muscles. The mammals and higher forms lack such bran- 

 chial muscles. Visceral mesoderm in this region gives rise to 

 the masticatory, facial, pharyngeal, and laryngeal muscles 

 as well as the heart. The hypobranchial muscles associated 

 with the branchial skeleton and the skeletal muscles of the 

 rest of the body are of somatic (myotome) origin (Figure 

 14-3). 



Embrvologically the locomotor muscles of the body arise 

 from the somites. The somites are segmental condensations 

 of mesenchyme, primarily myotome, enclosing a transient 

 part of the body cavity, the myocoel. The myotome arises 

 from that part of the somite ne.xt to the neural tube and noto- 

 chord. The adjacent outer wall of this part of the coelom is 

 the dermatome. Between the somite and the definitive 

 coelom is the nephrotome, and between the myotome and 

 the visceral part of the nephrotome is the sclerotome (Figure 

 10-15). Cells of the sclerotome move out of their area of 

 origin to enclose the neural tube and notochord. Mesen- 

 chyme cells of the other parts of the coelomic wall, except 

 the myotome, give rise to the smooth musculature of the 

 body. The origin of the somites and coelomic divisions in 

 .Amphioxus resembles events in the vertebrate (Figure 14-1). 



To produce the waves of contraction characteristic of 

 swimming, the myotomes tend to become V-shaped or W- 

 shaped (Figure 14-2). This is the first specialization dis- 

 played. In gnathostomes the myotome is divided by a hori- 



zontal septum into epaxial and hypa.xial divisions; the latter 

 extends down lateral to the coelom. 



The myotomes of fishes have become somewhat more 

 modified. In higher forms the myotomes have been variously 

 fused, subdivided, and resubdivided into a complex pattern 

 of vertebral and body-wall muscles; only the intercostal and 

 some of the muscles of the spinal column retain the segmen- 

 tal form, lying as they do between the ribs and vertebrae. 



With the origin of fins, a part of the myotomic muscula- 

 ture has migrated into the fins. This can be observed in the 

 shark where small buds from a large number of somites 

 grow into the base of the fin (Figure 14-3). The limb muscles 

 of tetrapods and the fin muscles of fishes differentiate from 

 simple dorsal and ventral masses. The ontogenetic course of 

 this subdivision will be described for the tetrapods. In the 

 tetrapods the limb musculature is not of apparent myotomal 

 origin but is derived from mesenchyme condensations whose 

 cells may be of myotomal origin. 



A modification of skeletal musculature occurs in the forma- 

 tion of electric organs. Such organs are obser\ed in the 

 Torpedo, a ray-like elasmobranch, and several families of 

 teleosts: the Mormyridae, Gymnotidae, Electrophoridae, 

 Uranoscopidae, and Malapteruridae. Generally the electric 

 organ is developed from the muscle of the tail, but in the 

 Stargazer, Aslroscopus (Family Uranoscopidae), the muscles 

 moving the eye are modified. In Malapterurus the subcutane- 

 ous (smooth) musculature covering the body behind the 

 head appears to have been transformed into this organ. 



The smooth musculature of the body is much like that seen 

 in the invertebrates. It lies in the skin and the walls of the 

 tubular organs; physiologically it is different from the loco- 

 motor muscles. The circular and longitudinal body-wall 

 muscles, so characteristic of invertebrates, are lacking in the 

 vertebrate. 



SKELETAL MUSCULATURE 



Discussion of the skeletal musculature of vertebrates is dif- 

 ficult because of the large number of separate elements in 

 this system, and the variations which these show. Muscles 

 range from separate entities with easily determined ori- 

 gins and insertions to those which are partly joined onto 



420 



