AMPHIBIA 285 



The heart of the frog beats for hours after being removed from 

 the body. Electrical, thermal and chemical stimuli will cause the 

 heart to beat again after it has ceased normal contractions. 



Vocal Organs. — In vertebrates the vocal and the respiratory- 

 organs are intimately associated since the production of sound is 

 caused by the expulsion of air from the lungs. The sound-producing 

 organs of the frog are situated in the larynx below the pharyngeal 

 cavity. The larynx opens into the pharynx through the glottis 

 above and posteriorly by a pair of openings into the lungs. The 

 skeleton of the larynx is composed of cartilages affording places for 

 attachment to muscles which open and close the glottis. 



Sound is produced by the expulsion of air from the lungs which 

 set in vibration the paired vocal cords. In the males of many species 

 of Rana we find a pair of vocal sacs at the side of the pharynx. 



Respiratory System. — The lungs are thin-walled sacs covered with 

 peritoneum. The inner surface of the lungs is divided by septa into 

 small chambers or alveoli which increase the amount of surface 

 exposed to the air. The walls of the alveoli are richly supplied 

 with capillaries. The alveoli are lined with a single layer of thin 

 flattened epithelial cells. On the edges of the septa they are colum- 

 nar and ciliated. Outside the epithelium is a layer of connective 

 tissue containing blood and lymph vessels and highly contractile 

 smooth muscle cells. 



The respiratory movements of the frog consist of throat move- 

 ments and lung breathing. Throat movements may take place 

 without movements of the body or nares, with the glottis closed and 

 no air passing into or out of the lungs. Air is drawn through the 

 open nares into the mouth cavity. Its floor is lowered, and forced 

 out through them when the floor is again raised. Lung breathing 

 causes distinct lateral expansions of the body wall. 



The process of respiration consists of: External respiration with 

 exchange of gas between the blood and the surrounding medium, 

 carbon dioxide being given off and oxygen taken in; and internal 

 respiration., gaseous exchange between the blood and the tissues. 

 Most of the oxygen in the blood is carried by red corpuscles in weak 

 chemical combination with hemoglobin. Oxygenated blood Is 

 bright red, impure blood is dark blue in color. 



The skin is a respiratory organ in air and water, and during 

 hibernation it is the only respiratory organ. More CO2 is given off 

 through the skin than through the lungs. 



