4 278 ZOOLOGY SECT. 



their size and appearance vary greatly according to their state 

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

 raised into a complex network of ridges abundantly supplied 

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

 laryngo-tracheal 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 



vt 



FIG. 931. Rana temporaria. The heart from the ventral aspect with the cavities laid open, 

 a, a', bristle in left carotid trunk ; au. v. v. auriculo-ventricular valves ; b. b', bristle in left 

 systemic trunk ; c, c', bristle in left pulmo-cutaneous trunk ; car. a. carotid artery ; car. gl. 

 carotid labyrinth; c. art. conus arteriosus ; car. tr. carotid trunk; /. au. left auricle; Iff. a. 

 lingual artery ; 1. r. longitudinal valve ; pul. cu. tr. pulmo-cutaneous trunk ; pul. v. aperture 

 of pulmonary veins; r. au. right auricle; s. au. ap. sinu-auricular aperture; spt. aur. 

 septum auricularum ; v, v.' valves; vt. ventricle 



vocal chords, by the vibration of which the croak of the Frog is 

 produced. 



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

 ing the floor of the mouth, draws air into the buccal cavity 

 through the nostrils. The floor of the mouth is then raised, the 

 \ostrils, which are valvular, are closed, and the air is forced through 

 .e glottis into the lungs. The skin also is an important respi- 

 dtory organ. 



Circulatory Organs. The pericardium (Fig. 929, pcd.} is not 

 situated in front of the general ccelome, as in Fishes, but lies in 



