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ABDOMINAL MUSCLES

In cartilaginous fish, the ancestral myotomes were separated into dorsal epaxial and ventral hypaxial divisions (Weichert, 1970).  In higher vertebrates, the hypaxial musculature is subdivided into a number of muscle groups such as separate layers of abdominal muscles and intercostals.  The external and internal oblique obliques laterally flex the abdomen.  (Webster, 1974; Weichert, 1970, p.512).
ABDOMINAL MUSCLES The rectus abdominis flexes the abdomen

.ABDOMINAL MUSCLES
The transversus abdominis is the deepest abdominal muscle; it (like all abdominal muscles) compresses the abdomen.

 
FROG

SALAMANDER

 SALAMANDER
Amniotes also develop a longissimus coli and levatores costarum from the hypaxial musculature (Webster, 1974; Weichert, 1970, p.512).

ALLIGATOR

INTERCOSTAL AND OBLIQUE MUSCLES

  
CHICKEN
ABDOMINAL MUSCLES
OPPOSSUM
ABDOMINAL MUSCLES ABDOMINAL MUSCLES
CAT
ABDOMINAL MUSCLES ABDOMINAL MUSCLES
ABDOMINAL MUSCLES

GOAT

ABDOMINAL MUSCLES

SHEEP

ABDOMINAL MUSCLES

COW

ABDOMINAL MUSCLES

pig

pig abdominal

MONKEY

ABDOMINAL MUSCLES

HUMAN MODEL

ABDOMINAL MUSCLES

 ABDOMINAL MUSCLES

In some fossil amphibians and in modern amniotes, the external and internal intercostal muscles develop from the external and internal obliques.  The intercostals muscles are better developed in mammals (Romer, p. 284).

ALLIGATOR

INTERCOSTALS

OPOSSUM

INTERCOSTALS

CAT

INTERCOSTALS

COW

INTERCOSTALS

 

 

HUMAN MODEL

INTERCOSTALS
In mammals, the diaphragm forms as a derivative of the rectus abdominis.  Its contraction is responsible for most of the change in the volume of the thoracic cavity during breathing (Romer, p. 290). 

OPOSSUM

DIAPHRAGM