chap, vi.] BRONCHI ; TRACHEA. 241 



extend into the cavity of the lung. The bronchi are 

 short indeed in lizards and snakes, but in crocodiles 

 and chelonians they extend for some considerable dis- 

 tance, and retain the cartilaginous rings by means of 

 which the tube is kept open ; these tubes give off 

 smaller lateral tubes, and so give rise to the so-called 

 bronchial tree, some of the branches of which lie 

 above and some below the pulmonary artery (p ; Fig. 

 102 A) ; these may be conveniently distinguished as the 

 eparterial and hyparterial bronchi. 



While this bronchial tree is comparatively simple 

 in Reptiles, it becomes much more complicated in Birds, 

 where both eparterial and hyparterial systems are 

 well developed and give off lateral branches, some of 

 which extend to the end of the lung. 



The difference between the bronchial tree of a Bird 

 and a Mammal does not lie in, as is ordinarily said, 

 the dichotomous mode of division of the bronchial 

 tubes of the latter, which never does obtain (Aeby), 

 but in the great reduction in the eparterial bronchial 

 system (Fig. 102 B), of which the right and left halves 

 are but rarely both present ; as a rule, the right is 

 lost, while in Hystrix both right and left eparterial 

 bronchi disappear. 



The trachea varies greatly in the extent and 

 characters of its development ; short in Amphibians, it 

 is of a considerable length in Reptiles, but in them the 

 cartilaginous rings are incomplete ; in Mammals the 

 rings are also always incomplete, but in Birds the sepa- 

 rate rings are not only complete, but tend to undergo 

 calcification, and in some cases, as in the Dinoriiis, 

 even ossification. The trachea is of great length in 

 birds, and while this may be often seen to be of 

 significance as an aid to the vocal organ (see page 391), 

 it has clearly the not unimportant function of forming 

 a long tube in which the air is slightly warmed before 

 it enters the lungs. In birds the lower, and in other 

 Q 16 



