44 VERTEBRATE RESPIRATION 



(a) VENTILATION IN THE FROG 



When a frog is observed at rest, movements of the throat are 

 visible at frequencies between 80 and 120 per minute (fig. 14). 

 These form the so-called Bucco-pharyngeal mowQinQnis. During 

 this activity the nostrils are partly open and the glottis closed 

 with the result that air contained in the bucco-pharynx is con- 

 stantly renewed. Gaseous exchange is thought to occur in the 

 vascularised mucous membranes and would assist in respiration, 

 but some authors have doubted whether its contribution is very 

 significant. The movements are produced by contractions of the 

 mylohyoideus muscle, which is a thin sheet of fibres running 

 transversely between the two rami of the lower jaw. 



Periodically these small fluctuations of the throat are inter- 

 rupted by quite large gulping movements which coincide with 

 movements of the flanks of the frog. The mechanism of this 

 pulmonary ventilation is essentially a buccal force-pump, involv- 

 ing changes in volume of the buccal cavity co-ordinated with the 

 operation of valves in the nostrils and the glottis. It may also be 

 assisted in some forms by active movements of the flanks. The 

 mechanism is illustrated diagrammatically in fig. 1 3 where four 

 main phases are recognisable. 



In the first stage (1), the glottis is closed and the nostrils open. 

 The floor of the mouth is lowered by the action of the sterno- 

 hyoideus muscle and air enters the bucco-pharynx because of 

 the reduction in pressure. In the next stage (2) the nostrils are 

 closed, the glottis is open and air is forced from the lungs into 

 the bucco-pharynx as the flanks contract, partly because of the 

 elasticity of the lungs. The eff*ects result in a passive extra lower- 

 ing of the floor of the mouth. The mixed air now contained in 

 the bucco-pharyngeal cavity is forced into the lungs (3) through 

 the open glottis ; this is brought about by contraction of the petro- 

 hyoideus muscles which raise the hyoid plate. The latter activity 

 may be repeated several times until finally (4), with the lung 

 filled, the glottis closes and the extra air is forced out through 

 the open nostrils. 



Variations on this plan have been described for different 



