RESPIRATION OF TETRAPODS 55 



of the respiratory cycle. One account (Zeuthen) believes that the 

 air passes through the parabronchi in opposite directions during 

 inspiration and expiration, but the other theory (Hazelhoff) 

 maintains that the direction of flow is the same throughout the 

 whole respiratory cycle and is in the direction from the posterior 

 secondary bronchi to the anterior group. The first theory seems 

 reasonable on aerodynamic grounds, for air coming down the 

 mesobronchus to the point of origin of the anterior secondary 

 bronchi has two possible paths in its passage to the abdominal 

 and posterior thoracic sacs. It can either continue along the 

 mesobronchus or pass via the anterior secondary, tertiary, and 

 posterior secondary system which run in parallel to the meso- 

 bronchial path. It is apparent that the volume of flow through 

 these parallel systems will be inversely proportional to the re- 

 sistances of the two parallel channels. If the flow is laminar, as 

 calculations indicate it probably is, and if the pressure gradient is 

 assumed to be the same across both systems, then, with Poi- 

 seuille's formula,* it is possible to calculate the ratio of flow 

 between them. It must be remembered that although the cross- 

 sectional area of each parabronchus is small there are a very 

 large number of them so that the total resistance to movement 

 along this path is not necessarily greater than that of the meso- 

 bronchus. Calculations show in fact that within a wide range of 

 variation of mesobronchial diameter there wifl always be con- 

 siderable flow through the parabronchi. During expiration, when 

 the posterior thoracic and abdominal sacs are compressed, air 

 will pass in the reverse direction along these two parallel paths 

 in the same proportion if the diameters of the parabronchi and 

 mesobronchi remain the same. Evidence for this type of flow 

 comes from determination of the CO 2 content of the air sacs 

 and the expired air, together with values for the volume of gas 

 entering and leaving the sacs. As the precise CO 2 content of the 

 mesobronchial air is not known, estimates were made based 



P 7T R^ 



* Rate of flow -= -^ , where P is the pressure difference along a 



8. L. 7} 

 rigid tube of length L and radius R. r] is the coefficient of viscosity. 



