6 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



drawn into the mouth. In the roof of the mouth are two pairs 

 of openings. The anterior pair, the inner nares, are the in- 

 ternal openings to the nose; the posterior pair, just posterior 

 to the eyeballs, are the internal openings to the wide Eustachian 

 tubes which lead to the mouth from the chamber of the ear 

 behind the tympanum. At the posterior end of the mouth is 

 the opening of the esophagus through which the food passes 

 into the stomach. Just below the opening of the esophagus 

 is a perpendicular slit-like opening, the glottis. This opens into 

 the short larynx through which the air passes to the lungs. 

 The flaps just within this opening are the vocal cords. 



Internal Structure. In making the dissection for the study 

 of the internal structure it is best to make the cut along the 

 ventral side from the anal opening, at the posterior end of the 

 body, to the angle of the lower jaw. The first cut should be 

 made only through the skin in order to expose some of the 

 muscles that control the movements of the body. The large 

 sheet of abdominal muscles covering the ventral side of the 

 frog consists of two sets, an outer and an inner layer. Poste- 

 riorly they are attached to the bony pelvic girdle which sup- 

 ports the hind legs. The size, position and points of attach- 

 ment of the heavy bundles of muscles that control the leg 

 movements should also be noted. 



The incision may then be made through the body-wall, and 

 the sides fastened back to expose the internal organs. The 

 digestive system may be studied first. The esophagus, as 

 we have noted, leads from the opening at the back of the mouth 

 to the stomach. The stomach is somewhat crescent-shaped, 

 and lies mostly on the left side of the body. The anterior, or 

 cardiac, end is larger than the posterior, or pyloric, end where it 

 joins the small intestine. The small intestine is a long coiled 

 tube opening into the large intestine or rectum. The posterior 

 part of the rectum is known as the cloaca, for it also receives 

 the waste products from the bladder and kidneys. The ova 

 and spermatozoa also pass from the body through the cloaca. 

 The reddish-brown lobes of the liver are conspicuous. They 

 secrete the bile, which is an alkaline fluid that aids in digestion. 

 The gall-bladder, where the bile is stored, lies between the lobes 



