i68 THE BIOLOGY OF THE FROG chap. 



mains closed. When air is let out of the lungs, they shrivel 

 to an inconspicuous size. The inner surface of the lungs is 

 divided by a network of septa into a series of small cham- 

 bers or alveoli, by means of which the amount of surface 

 exposed to the air is very greatly increased. The walls of 

 the alveoli are richly supplied with blood vessels which break 

 up to form a fine capillary network. The inner surface of 

 the alveoli is covered with a single layer of epithelial cells 

 which are very thin and flattened except on the edges of the 

 septa, where they become cylindrical and ciliated. Outside 

 the epithelium is a connective tissue layer whiq^h contains 

 the blood and lymph vessels, and numerous unstriated 

 muscle cells which give the lungs their great power of con- 

 traction. The outer surface of the lungs is coated with 

 peritoneum. 



The area of the inner surface of the lungs of Rana escu- 

 lenta has been carefully calculated by Krogh. In a speci- 

 men weighing 40 g. it was found to be 98 sq. cm. The 

 total surface of the skin was estimated to be 154 sq. cm. in 

 the same specimen. 



The Respiratory Movements. — Since the frog has no 

 ribs, it is unable to draw in air by enlarging the cavity con- 

 taining the lungs as the higher animals do, and it has 

 recourse, therefore, to a more indirect method of inspiration. 

 If one watches the respiratory movements of a frog, it will 

 be seen that the floor of the mouth rises and falls at quite 

 regular intervals. Usually at somewhat greater intervals 

 there may be seen a contraction followed by a sudden 

 expansion of the body wall ; and accompanying the latter 

 movement there is a brief closure of the nares. The respi- 

 ratory movements of the frog fall into two classes : (i) the 

 oscillatory throat movements, and (2) the movements directly 

 concerned in filling and emptying the lungs. The throat 



