The Frog: A Representative Vertebrate 63 



These two combinations of action force the air through the glottis 

 into the larynx or voice box. From the larynx, the air passes through 

 two short tubes, the bronchi, into the lungs. The air is forced out of 

 the lungs by contractions of the muscles of the body wall. 



In the larynx there are two elastic bands, the vocal cords. When 

 air is forced out of the lungs over these bands, they vibrate to pro- 

 duce the typical croaking noise of the frog. In the male, the special 

 vocal sacs add resonance to this croak. 



The lungs are so inefficient that the frog would soon suffocate if 

 it depended upon them entirely for respiratory exchange. Most gase- 

 ous exchange takes place through the thin moist skin. The moist 

 inner lining of the frog's mouth is also used for this exchange. Much 

 of the air coming into the mouth never passes into the lungs, rather 

 it simply goes in and out of the nares with the moist membranes of 

 the mouth acting as the medium of exchange. 



THE CIRCULATORY SYSTEM 



The delivery of oxygen and food to all parts of the body and the 

 removal of waste products is made possible by the circulatory system. 

 The blood is the medium of this transport, and is carried through 

 the body in a series of tubes. The heart is the central pumping 

 mechanism. 



The Heart. — The heart is located in the anterior part of the 

 coelom and is surrounded by the thin, but tough, pericardium. It is 

 composed of a series of connected chambers: on the dorsal portion 

 of the heart is the triangular, thin-walled sac, the sinus venosus; on 

 either side of the heart are the paired thin-walled auricles which are 

 separated from one another by a median septum; the most conspicu- 

 ous structure is the heavy, muscular ventricle from which the truncus 

 arteriosus originates. 



Between these chambers are various valves which prevent the 

 backflow of blood. Between the auricles and the ventricles are the 

 auricular-ventricular valves. The work of these valves is made possible 

 by a series of special papillary muscles on the walls of the muscular 

 ventricle. These are attached by long cords, the chordae tendineae, 

 to the valves. The base of the truncus arteriosus is guarded by a 

 valve consisting of three pocketlike flaps, the semilunar valves; within 

 the truncus itself is a special valve, the spiral valve, which has the 

 function of directing the blood as it leaves the heart. 



