94 VERTEBRATE ZOOLOGY 



elliptical elevation formed by the two arytenoid cartilages ; it is 

 usually closed, but may be opened with a needle. Place the end 

 of a blowpipe in it and blow up the lungs. 



The frog has two methods of respiration,— ( i ) with the skin 

 and with the mucous membrane of the mouth and pharynx, and 

 (2) with the lungs. Air is taken by regular inspirations through the 

 nostrils into the mouth and pharynx, where it is acted upon by the 

 highly vascular mucous membrane. By an act of swallowing it 

 is also taken, at irregular intervals, through the glottis into the 

 lungs. It is expelled from the lungs by the elasticity of their walls, 

 which contain muscle fibers, and by that of the muscular sides of 

 the body. Immediately after the expiration, air is again swallowed, 

 so that the lungs are kept filled. The floor of the mouth will be 

 observed in the live frog to oscillate rapidly and regularly. This 

 act is not connected directly with the pulmonary but rather with 

 the pharyngeal respiration. 



The opening behind the glottis into the digestive tract is the 

 gullet, or oesophagus. Probe it. 



In the roof of the mouth note the upper jaw, in which is a row 

 of teeth called the maxillary teeth. Just behind them in the for- 

 ward part of the mouth, near the median line, are two small groups 

 of teeth called the vomerine teeth. On each side of these is one 

 of the paired internal nares, the inner openings of the nostrils. 

 Probe them. Near the angle of the mouth on each side is 

 a large opening into the tympanic cavity, the Eustachian tube. 

 Probe one. 



Exercise 4. Draw a sketch of the opened mouth and pharynx on a 

 scale of about 2, and carefully label all the organs above mentioned. 



The Internal Organs. Place the animal on its back in a dissect- 

 ing pan containing water, with its head away from you, and pin it 

 fast with a large pin through the tip of the jaw and one through 

 each of the four legs. Raise the skin of the belly with forceps, 

 and with scissors make an incision in it along the midventral line 

 the entire length of the body. 



Notice the looseness of the skin and the large space between 

 it and the underlying muscles. This space is a lymph cavity. Note 



