RESPIRATION OF TETRAPODS 



49 



the lungs being filled by the action of a costal pump. Ribs are 

 absent in modern Amphibia although they were found in their 

 mesozoic ancestors but they are universal among reptiles. 

 Movement of the ribs is produced by the intercostal muscles 

 which expand the cage in the thorax and produce a reduction 

 of the pressure within the lung. Inspiration is therefore active 

 and a suction pump is the main mechanism ventilating the lung. 

 Expiratory movements are partly passive but may be assisted 

 by contraction of transverse abdominal muscles and the smooth 

 muscles of the lungs themselves. 



More detailed studies of ventilation in lizards have led to the 

 description of a three-phase sequence of movements (fig. 15). 



Pause 



E, 



Lung full and under 

 slight pressure 



Open Close 



Fig. 15. 



Diagram to show the triphasic nature of the respiratory move- 

 ments of a lizard {Lacertd). Upwards on the thoracic mechanogram 

 is produced by an inwards movement. (Based on Boelaert, K.\Arch. 

 int. Physiol Vol. 51, 1941.) 



This can be seen in many lizards where respiration begins with 

 an initial expiratory phase followed rapidly by an inspiration 

 and finally a brief expiratory movement. A pause ensues before 

 the next respiratory act. During the pause the lung is inflated 

 and the glottis closed so that gaseous exchange continues. The 

 gas contained in the lung is held at a pressure which exceeds 

 atmospheric; the initial expiratory movement is an active one 



