MUSCULAR SYSTEM 



135 



muscles, or transversely into two successive portions. On the other 

 hand, two muscles, diiferent in origin, may fuse; while, with loss of 

 function of a part, its muscles may degenerate or entirely disappear. 

 Muscles may wander far from their point of ontogenetic origin and 

 become connected with parts widely remote, a condition strikingly 

 illustrated in the facial muscles of the higher mammals, where nerve 

 supply still shows the original history. 



In the ichthyopsida the trunk muscles clearly show their myo- 

 tomic origin, but even here there are tendencies to division and spe- 

 cialization. The ventral muscles on either side of the body cavity 



hypn 



Fig. 145. — Diagram of muscle segments in head of embryo vertebrate, based upon 

 a shark, after Neal. The anterior myotomes tend to divide into dorsal and ventral 

 moieties; persistent myotomes lined, transient with broken lines; central nervous 

 system dotted, nerves black, a, premandibular somite; abd, abducens nerve^ hyp, 

 hypoglossal musculature; hypn, hypoglossal nerves; ow, oculomotor nerve; sp, spiracle; 

 1-6, first six somites (4, 5, 6, functional in Petromyzon); I-VIII neuromeres. 



of the amphibia (fig. 147) are divided into a lateral oblique and 

 a medial rectus system, the rectus muscles of the two sides being 

 separated by the linea alba already referred to. The rectus muscles, 

 in turn, become divided into successive groups, a rectus abdominis 

 in the abdominal region, extending from the pelvis to the sternum; a 

 Sternohyoid from the sternum to the hyoid bone, and a geniohyoid 

 from the hyoid to the tip of the lower jaw. The obHque region 

 is also divided from the exterior to the peritoneal side, into three 

 layers (obliques and transversus) characterized by the direction of 

 the fibres. In the higher vertebrates, with the appearance of well- 

 developed ribs, the oblique muscles furnish the two layers of inter- 

 costal muscles, extending from rib to rib, and in front of the ribs they 

 form the scalene muscles, extending from the ribs along the side of 

 the neck. In the non-placental mammals a strong pyramidalis mus- 



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