STRUCTURE OF THE VERTEBRATES 



185 



muscles are in three la^-ers, (1) an inner transverse group with 

 the fibers running dorso-ventrally ; (2) the middle oblique fibers 

 which are directed in an antero-ventral direction; and (3) the 

 external oblique fibers which pass from the epaxial line in a 

 postero-ventral direction. The fibers from either side meet a con- 

 nective tissue septum, the linea alba in the midline. The rectus 

 abdominis muscle is another part of the abdominal group. The 

 fibers are longitudinal, arising on the pubic bones and extending 



Spine of vertebra 



paxial muscles 



Hypaxial 



{abdominal) 



Mtiscles 



Rectus abdominis 

 Linea alba 



Fig. 105. Cross Section of a Mammal through the Lumbar region. The 

 skin and viscera are removed, with the muscle groups in position. 



anteriorly to the ribs, where they insert in a wide band. The 

 hypaxial caudal muscles, like those of the epaxial region, undergo 

 degeneration during their evolution, and are left in man as a 

 few small muscles in the anal region. 



D. Branchiomeric IMuscles 



These striated muscles which develop from the hypomere 

 were originally attached to the gill bars. As the anterior two 

 pairs of visceral arches developed into the jaws and hyoid ap- 



