60 The Vertebrate Organ Systems 



Movement of parts can be brought about only by contraction ; thus 

 muscles work in pairs known as antagonistic groups. If one group of 

 muscles bends a joint, another must straighten it. 



Inasmuch as the muscular system of mammals is discussed in 

 detail later, it will be sufficient here to point out some of the more 

 conspicuous muscles of the frog. 



Along the ventral side of i:he trunk is the large rectus abdominis. 

 It is opposed in its action by the large muscle of the back, the longis- 

 simus dorsi. These two muscles are important in maintaining the 

 typical posture of the frog. The rectus abdominis extends along the 

 entire ventral surface. When it contracts, the head is bent downward 

 and forward. The longissimus dorsi, running along the backbone from 

 the skull to the sacrum, contracts to bend the head and shoulders 

 upward. A long sheet of muscle extends along the sides of the body 

 from the back of the forelimbs. This is the external oblique. When 

 these three muscles act together, croaking, egg laying, etc., are made 

 possible. 



GENERAL INTERNAL ANATOMY 



If the ventral surface of the frog is slit open, the large body cavity 

 or coelom is visible. This coelom is lined with a smooth, shiny mem- 

 brane, the peritoneum. That portion of the peritoneum lining the body 

 wall is the parietal peritoneum, and that covering the internal organs 

 is the visceral peritoneum. This peritoneum is continuous with the 

 two regions being connected by double layers of the same tissue which 

 are known as mesenteries. These mesenteries support the internal 

 organs. 



Within the coelom are many different organs which are collec- 

 tively known as the viscera. The anterior portion of the coelom is 

 occupied by the heart and lungs. Surrounding the heart is a thin, 

 tough membrane, the pericardium. Posterior to the paired saclike 

 lungs is the large, lobed liver with the saclike gall bladder between the 

 right and middle lobes. The gall bladder is dark green in color due 

 to the bile stored in it. 



The greater portion of the digestive tract is easily seen. Dorsal 

 to the heart is the short esophagus which enters the elongated curved 

 stomach. Leading from the stomach is the long, coiled, slender small 

 intestine which finally enters the short wide large intestine. Near 

 the stomach is the slender white pancreas, and just behind it is the small 



