708 THE MUSCULAR SYSTEM 



hypaxial musculature of the shark is disrupted, and the myotomes tend to 

 spHt into layers or sheets of muscles. This splitting is slight in Necturus and 

 marked in the frog. Also, in the frog, the myocommata are displaced as a 

 part of the muscular-skeletal mechanism, with the exception of the rectus 

 abdominis muscle whose segmentation possibly is a secondary development. 



In mammals (fig. 327E), the epaxial musculature is differentiated into 

 a complex of muscles, extending from the sacral area anteriorly into the cervical 

 region and connecting the various vertebrae with each other and the vertebral 

 column with the ribs. The epaxial musculature in the trunk area of the bird is 

 much less developed than it is in the mammal. The hypaxial musculature 

 in both bird and mammal becomes separated into distinct layers, such as 

 the external, internal oblique, and transversus muscles. External and internal 

 intercostal muscles are present between the ribs. In the midventral area, the 

 rectus abdominis muscle tends to retain its primitive segmentation. 



It is noteworthy to observe that the external and internal intercostal muscles 

 in the mammal appear much the same as the lateral body muscles in Necturus, 

 particularly if we keep in mind the fact that ribs grow out into the myoseptal 

 (myocommal) area (fig. 326D). The external intercostal muscles run postero- 

 ventrally, while the internal intercostals pass antero-ventrally from one rib 

 to the next (fig. 327E). The intercostal musculature of the mammal thus 

 retains the primitive, segmented condition. 



c) Aerial Adaptations. The musculature of the bird is a highly dif- 

 ferentiated organization of structures in which the primitive myotomic plan 

 is greatly distorted. The epaxial musculature is reduced greatly over the trunk 

 region, although well developed in the cervical area. Hypaxial musculature 

 is present in the form of external and internal oblique, and transverse muscle 

 layers. Very short rectus abdominis muscles are to be found. Aside from the 

 intrinsic muscles of the limbs, a large percentage of the volume of the hypaxial 



Fig. 327. Development of branchial and somitic muscles in various vertebrates. (A) 

 Basic areas of the embryo from which skeletal muscle develops. The skeletal muscles of 

 the limb buds are portrayed as masses of mesenchyme represented in this figure as stippled 

 areas in the two limb buds. The origin of this mesenchyme varies in different vertebrates 

 (see text). (B) Skeletal muscular development in the shark. The muscle tissue derived 

 from the hyoid visceral arch is shown in black with white lines. Muscle tissue derivatives 

 from the mandibular visceral arch are shown anterior to the black-white line areas of the 

 hyoid musculature. (C) Same for Necturus inuculosus. (D) Same for the frog. (E) 

 Epaxial muscles and intercostal part of hypaxial muscles of cat. External intercostals 

 mostly removed. The "masseter muscle," a derivative of the mandibular visceral arch tissue 

 of the embryo, also is shown. (E') Superficial facial and platysma muscle distribution in 

 the cat. These muscles are derivatives of the hyoid visceral and mesenchyme. (E") 

 External pterygoid muscle in the cat, another derivative of the branchial arch mesen- 

 chyme. (F) Anterior muscles of the goose. The muscles derived from the primitive 

 hyoid visceral arch are shown in black with white lines. (Adapted from Huber, 1930, 

 Quart. Rev. Biol., vol. 5, and from FiJrbringer, 1888, Morphologic und Systemalik der 

 Vogel, van Holkema, Amsterdam.) (F') The temporal and masseter muscles in the 

 common fowl. These muscles are derived from the mandibular visceral arch. 



