THE FROG 165 



jaws? Where? Of what value are the teeth? Does the 

 frog masticate its food? The back part of the mouth, the 

 region of the throat, is caled the pharynx. Pass a bristle 

 into the nostrils and see where it enters the mouth. Make 

 an opening in the tympanic membrane and insert a bristle 

 through the mouth. This is the eustachian tube. With 

 the bristle find an opening into the esophagus and a narrow 

 opening at the front into the bronchial tube. Make a list of 

 the openings that communicate with the pharynx and state 

 briefly the use of each. 



This prepares us for a study of those systems that are 

 centered in the protected thoracic cavity. 



Internal Structure. — I. The Thoracic Cavity. — (i) 

 It is well to dissect the frog under water or in a very weak 

 solution of formalin. Pin the frog securely on the dorsal side. 

 With scissors or scalpel cut along the midventral line sev- 

 ering the thin outer skin from the posterior end of the body 

 to the mouth. At right angles to this line cut on both sides 

 under the front limbs, turn the flaps back as far as possible 

 and cut them off, thus exposing the body wall, the abdom- 

 inal muscles. Note the arrangement of muscular tissue and 

 follow them both ways to their places of attachment. What 

 effect would their contraction have? What position would 

 that give the body? This muscle is called rectus abdomin- 

 alis. Note the broad fan-shaped muscle running from the 

 central portion of the breast bone to the front limb. It is 

 called the pectoralis. What is 'the function of this muscle? 

 How is its work performed? Look in the muscular walls 

 for evidences of circulation of blood. Follow the blood ves- 

 sels as far as you can. Do you find many blood vessels going 

 to the surface? Of what use can they be at the surface? 



