RESPIRATION OF TETRAPODS 51 



embedded dorsally in the ribs. The bronchial tubes communi- 

 cate, however, with air sacs which take up a large volume of the 

 body cavity and extend into the cavities of the long bones. These 

 air sacs are not concerned with gaseous exchange as such, for 

 they have a relatively meagre blood supply and carbon monoxide 

 injected into an air sac which has been ligatured from the lung 

 does not get into the general circulation and produce poisoning 

 of the haemoglobin. The precise function of the air sacs remains 

 doubtful although they were first described by William Harvey 

 in 1651. Table 8 gives the percentage of the inspired air which 

 is contained in the lung and different air sacs of the hen. It is 

 apparent that the abdominal sacs are by far the largest cavity of 

 the whole respiratory system, taking between 70 and 90 % of the 

 total inspired air. The volume of air inspired or expired in 

 one breath (tidal air) is about 35 to 45 ccs in a duck or chicken 

 and 5 ccs in a pigeon. These figures are only 10 to 15% of 

 the total capacity of the respiratory system and hence only a 

 small portion of the air in the sacs must be expired at each 

 respiration. 



Table 8 

 Percentage of inspired air entering different parts of 

 the respiratory system of a Hen (after Zeuthen, 1942) 



The trachea is supported by cartilaginous rings and at its base, 

 before dividing into the two primary bronchi, the voice box or 

 syrinx is found (fig. 16a). Each primary bronchus continues 

 through the middle of the lung as the single mesobronchus {\ mm. 

 in cross section), ending posteriorly in an abdominal air sac. 

 From this primary bronchus arise two groups of secondary 

 bronchi. The anterior group arise on the dorsal side of the 



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