THE RESPIRATORY APPARATUS IN BIRDS. 559 



Beyond this enlargement, it diminishes in capacity by the emission of branches, 

 losing its cylindrical form to assume that of a cone with a trmicated summit. 

 Its walls are almost entirely destitute of cartilaginous rings, so that the origin of 

 the principal conduits is constantly membranous. 



" The air-passages arising from this common trunk to constitute the frame- 

 work of the lung, are remarkable for their uniformity in number, form, and the 

 direction they olTer in all classes of birds. They are generally twelve, and their 

 origin is thus distributed : four arise from the internal wall of the trunk by a 

 series of orifices placed one after the other ; seven are detached from its external 

 wall by a second series of orifices also disposed in rows ; the twelfth springs from 

 its inferior wall, and immediately bends downwards and outwards to open into 

 the posterior diaphragmatic reservoir, which may be considered as a terminal 

 branch of the principal trunk. 



" All the canals which have their origin from these linear series of openings 

 on the internal and external walls of the generating trunk, show this common 

 disposition : that from their commencement they pass towards the periphery of 

 the lung, that they divide and subdivide at this periphery, that they cover it 

 with their ramifications, and do not leave it to enter the pulmonary parenchyma 

 until their volume has been considerably reduced. 



" The conduits leaving the orifices situated on the inner wall of the a3rial 

 trunk ramify on the inferior face of the lung ; those proceeding from the eche- 

 loned orifices on the outer wall are distributed on the opposite face. The first 

 constitute the diaphragmatic, and the second the costal bronchial tubes. 



" The diaphragmatic hroiichial tubes, four in number, like the orifices from 

 which they originate, may be distinguished by the numerical names of first, 

 second, third, and fourth, in proceeding from before to behind ; the first bronchus 

 is carried forward horizontally, the second transversely inwards, the third 

 obliquely inwards and backwards, and the fourth directly backwards. In view 

 of their divergent direction, which resembles a fan, they might be designated as 

 the anterior, internal, and posterior diaphragmatic bronchial tubes ; and to dis- 

 tinguish the last two, the more voluminous one — which is directed backwards and 

 inwards — might be named the great posterior diaphragmatic bronchus, and the 

 one passing directly backwards, the small posterior diaphragmatic bronchus. 



" The costal bronchial tubes, seven in number, may be also designated as first, 

 second, third, etc., in proceeding from before to behind. Parallel at their origin, 

 and in juxtaposition, like the pipes of an organ, they separate after following a 

 certain course, and affect, by their divergence, the fan-shape already observed in 

 the disposition of the diaphragmatic bronchi. Like the latter, they become 

 peripheral from their origin, and spread out from centre to circmnference. The 

 first is carried very obliquely upwards and inwards, to attain the anterior 

 extremity of the lung ; all the branches it furnishes arise from its anterior wall, 

 and those which are nearest its origin are inflected to gain the external border 

 of the organ. The succeeding tubes are directed forwards, the others forwards 

 and inwards ; while all proceed to meet those coming from the anterior dia- 

 phragmatic bronchus, though they do not anastomose with them. Coming in 

 contact, they plunge into the pulmonary tissue in such a way, that, when a lung is 

 inflated, we observe between these two orders of ramifications a very manifest 

 groove, which is perfectly distinct from those due to the protrusion of the ribs ; 

 this groove evidently represents, though in a rudimentary state, the inter- 

 lobular fissures in the lungs of quadrupeds. 



