282 



THE ALIMENTAEY TRACT, ETC. 



Muscles of the tongue, from the ventral 



surface. 



G M. genio-e;los8us. 

 G// Straight fibres of the M. genio-glossus 

 G//1 Curved fibres of the M. genio-glossus. 

 ffy and H>A M. hyoglossus. 

 Z Borders of the tongue. 



The 3f. In/oij/oxxux is the retractor 

 of the tongue, the M. genio-gtoxsn* 

 the protractor. 



(For mucous membrane of the 

 tongue, see organ of taste. The 

 vocal sacs are described with the 

 organs of voice and respiration.) 



B. The Oesophagus and Stomach 



(Figs. 184, 185, 189, 194, 195, 

 199). 



a. General description. The 



oesophagus is not separated from the 

 pharyngo-oral cavity by a sharp line 

 of demarcation. It has a length of 

 only a few mm. in medium- sized 

 frogs (Fig. 184 Oe), and lies in the 

 middle line of the body, supported 

 on either side by the cornua of the 

 hyoid bone. The oesophagus lies on 



the dorsal wall of the larynx, is smooth externally, and is thrown 

 into well-marked longitudinal folds internally. 



The transition from oesophagus to stomach is somewhat indefi- 

 nitely marked by a slight dilatation of the tube, often scarcely per- 

 ceptible, still it can always be recognized by an abrupt curvature to 

 the left (Fig. 184 M), which becomes more prominent when the 

 stomach is distended. Externally the stomach is seen as an elon- 

 gated, slightly curved cone, smooth externally and of equal diameter 

 throughout. 



The oesophagus and stomach are held in position by folds of 

 peritoneum (mesentery), which attaches them to the dorsal wall of 

 the abdominal cavity, to the lungs, pericardium, and liver ; and by 

 the blood-vessels. 



b. Minute structure. The walls of these viscera are composed 

 of four layers or coats. 



(1) The serous coat (Fig. 186) is a layer of endothelium, lying 

 on a very thin stratum of connective-tissue; the whole derived 

 from the peritoneum, which encloses the organs. 



(2) The muscular coat (Fig. i 86 LN and TA1) is arranged in 



