i68 PHYLUM CHORDATA 



single space which extends back to the beginning of the oesopha- 

 gus. With forceps pull the tongue forward ; it is a slimy, band- 

 like structure which is attached only at its forward end. The 

 hinder end, which extends back into the pharynx, is bilobed. 

 The lower jaw is without teeth. Just back of the tongue in the 

 floor of the mouth may be felt the hyoid cartilage, which supports 

 the tongue. 



Back of the tongue is the glottis, a median, longitudinal slit 

 which opens into the lungs. The glottis is in the middle of an 

 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 

 the mucous membrane of the mouth and pharynx, and (2) with 

 the lungs. Air is taken by regular inspirations through the nos- 

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

 highly vascular mucous membrane. It is also at irregular inter- 

 vals taken by an act of swallowing through the glottis into the 

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

 walls, which contain muscle fibers, and 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 inner openings of the nostrils. Probe them. Near the 

 angle of the mouth on each side is the large opening into the tym- 

 panic 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 mentioned 

 above. 



