184 



STRUCTURE OF THE VERTEBRATES 



clearly delimited from the body. In the higher anthropoids and 

 man, in which the tail is vestigial, the caudal muscles are left 

 as a small group of anal muscles, giving added support to the 

 perineum in the upright walking position. 



Certain of the epaxial muscles migrate to the ventral side of 

 the vertebral transverse processes and pass posteriorly to the 



Occipital mmcles 

 Irr— Spines of vertebrae 



Spinal muscles 



Ribs 



Costal muscles 



Sacro-spinales 



Fig. 104. Back Muscles of Man. The superficial mviscles are shown dis- 

 sected away. The left side of the figure shows muscles which are deeper 

 in position than those on the right side. 



pelvis and femur. This is the powerful ilio-psoas group, its homo- 

 logue beginning in the urodele amphibia. 



During the course of evolution the hypaxial region remains 

 more primitive. The ventral elements of the myotome form the 

 costal, or rib, muscles which are distinctly metameric. In most 

 vertebrates they are limited to the thoracic region of the body. 

 In the abdominal region the hypaxial muscles form the layers 

 of the body wall and the rectus abdominis. The abdominal 



