IV THE INTERNAL STRUCTURE 71 



specimen of R. pipiens I have found as many as six hundred 

 and forty. 



The tongue of the frog can be readily thrown out of the 

 mouth, as it is in capturing an insect, and withdrawn again 

 with great quickness. The sticky secretion with which it 

 is covered causes it to adhere to insects or other prey with 

 which it comes in contact. The victims are then drawn 

 back into the mouth, where the tongue may assist in pushing 

 them back into the throat, where they can be swallowed. 

 The sticky substance on the frog's tongue is not produced 

 by the mucous glands, but is derived, in part at least, if not 

 entirely, from the intermaxillaiy glajid, which lies above the 

 anterior part of the roof of the mouth. This gland is partly 

 mclosed by the premaxillary bones just in front of the nasal 

 cavities. It really consists of an aggregate of several small 

 glands (twenty to twenty-five in Rana escidenta), with 

 as many independent ducts leading into the cavity of the 

 mouth. Wiedersheim has shown that the secretion of these 

 glands is remarkably adhesive. 



The mouth cavity in general is lined by a mucous mem- 

 brane which varies considerably in structure in different 

 regions. Posteriorly it is thrown into folds which converge 

 toward the esophagus. The epithelium which forms the 

 superficial portion of this membrane is ciliated over a large 

 part of the mouth, and there are numerous goblet cells 

 scattered about among the others. The action of the cilia 

 may be demonstrated in a live or recently killed frog by 

 scattering powdered carmine over the roof of the mouth. 

 The carmine grains will be seen to be carried very slowly 

 backward, and eventually they will be drawn into the esoph- 

 agus. The whole membrane of the mouth takes part to a 

 greater or less extent in the production of mucus. 



The Teeth. — The teeth of the frog are very numerous, 



