416 MANUAL OF ZOOLOGY SECT. 



anterior part of the coelome above the heart and liver ; their 

 size and appearance vary greatly according to their state of 

 distension. Each contains a spacious cavity, and has its walls 

 raised into a complete network of ridges abundantly supplied 

 with blood-vessels. The two lungs open anteriorly into a 

 small laryngo-trachcal chamber which communicates with the 

 mouth by the narrow slit-like glottis. The walls of the 

 laryngo-tracheal chamber are supported by a cartilaginous 

 framework, and its mucous membrane is raised into a pair of 

 horizontal folds, the vocal chords, by the vibration of which 

 the croak of the Frog is produced. 



In breathing, the Frog keeps its mouth closed, and, by 

 depressing the floor of the mouth, draws air into the buccal 

 cavity through the nostrils. The floor of the mouth is then 

 raised, the nostrils are closed, and the air is forced through 

 the glottis into the lungs. The skin also is an important 

 respiratory organ. 



The pericardium (Fig. 235, pcd.) is not a separate chamber, 

 as in Fishes, but the heart, enclosed in a pericardial mem- 

 brane, lies in the general ccelomic cavity between the gullet 

 above and the epicoracoids below. The heart consists 

 of a sinus venosus (Figs. 235 and 236, s. z>.), right and 

 left auricles (r. au., I. au.), a ventricle (v., vf.), and a 

 conus arteriosus (c. art.). As in Dipnoi, the sinus venosus 

 opens into the right auricle, the pulmonary veins into the 

 left ; a striking advance is seen in the greatly increased size 

 of the left auricle and its separation by a complete partition, 

 the septum auricularum (Fig. 236, spt. aur.), from the right. 

 The two auricles open by a common auriculo-ventricular 

 aperture, guarded by a pair of valves (au. v. v.), into the sin- 

 gle ventricle. The conus springs from the right side of the 

 base of the ventricle ; it is separated from the latter by three 

 small semilunar valves, and is traversed obliquely along its 



