710 



THE MUSCULAR SYSTEM 



musculature of the bird is contained within the pectoral muscles (fig. 327F). 

 As such the pectoral musculature represents an extreme adaptation to the 

 flying habit. A somewhat similar adaptation is found among mammals, in the 

 bat group. Myotomic metamerism is much less evident in the bird than in 

 any other group of vertebrates, and the only remains of it appear in the 

 intercostal muscles and some of the deeper muscles of the cervical area. 



2) Development of Trunk and Tail Musculature: a) General Features 

 OF Myotomic Differentiation in the Trunk. The muscles of the trunk 

 are derived from the primitive myotomes. As described previously, Chapters 

 11, 12, and 15, the primitive body segment or somite differentiates into the 



HYOID SWELLING 



EXTERNAL E 



MANDIBULAR PROCESS 

 MAXILLARY PROCESS 



4TH CERVICAL MYOTOME 



ARM MUSCULATURE 



PRECARTILAGE 

 PRIMORDiUM OF 

 ARM SKELETON 



ST THORACIC MYOTOME 



MESENCHYMAL 



PRIMORDIUM OF 



PECTORAL MUSCLES 



VENTRAL (HYPAXIAL) REGION 



OF 7th myotome 



DORSAL (EPAXIALI REGION 



OF tth myotome 



1ST LUMBAR MYOTOME 

 SPINAL GANGLION 



LUMBOSACRAL PLEXUS 



ST SACRAL MYOTOME 



9MM 

 4 1/2 WEEKS 



Fig. 328. Muscle development in the human embryo. (A and B redrawn from Bardeen 

 and Lewis, 1901, Am. J. Anat., 1.) (A) Early division of truncal myotomes into dorsal 

 (epaxial) and ventral (hypaxial) regions. 



