XII PHYLUM CHORDATA 419 



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 is also an important 

 respiratory organ. 



The pericardium (Fig. 251, pcd) is not a separate cham- 

 ber, as in fishes, but the heart, enclosed in a pericardial 

 membrane, lies in the general ccelomic cavity between the 

 gullet above and the epicoracoids below. The heart con- 

 sists of a sinus venosus (Figs. 251 and 252, s.v), right 

 and left auricles (r. au, I. au), a ventricle (v,vt), 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. 252, 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 

 whole length by a large flap-like longitudinal valve (hi) 

 which springs from its dorsal wall and is free ventrally. The 

 conus passes without change of diameter into a bulbas aorta, 

 the two being separated by a semilunar valve and by the 

 free end of the longitudinal valve. The bulbus gives off two 

 branches, right and left, each of them divided by two longi- 

 tudinal partitions into three vessels, — an inner or anterior, 

 the carotid trunk (car. tr), a middle, the systemic trunk or 

 aortic arch, and an outer or posterior, the pulmo-cutaneous 



