560 TRE RESPIRATORY APPARATUS IN BIRDS. 



" The second, third, and fourth costal bronchi follow a transverse course, 

 and ramify on the inner border of the lung ; the fifth and sixth incline towards 

 the posterior extremity of the organ ; the seventh, very small, reaches this 

 extremity and disappears. 



" The first costal bronchus is the most voluminous ; those succeeding it 

 gradually diminish in calibre. At their point of emergence they adhere closely 

 to the ribs ; all are imperforate, and this feature essentially distinguishes them 

 from*those occupying the opposite face. 



" The canaUcuU furnished by these principal tubes do not sensibly differ in 

 calibre in the various bronchi ; all offer an equal diameter, and their dimensions 

 are only in relation to the total volume of the lung. All are detached at a right 

 angle from the pulmonary wall of each bronchus, and descend pei^iendicularly into 

 the lung ; and all, from their origin to their termination, preserve the -same 

 diameter, and consequently the same cylindrical form. If this mode of ramifica- 

 tion be compared with that observed in Mammals, it will be seen to differ con- 

 siderably. In the latter class, the air-passages affect the dichotomous division 

 proper to the arteries and veins, the result of which is a series of arborescent canals 

 decreasing in capacity. In birds only two kinds of conduits are observed, the 

 primitive and peripheral, disposed around a generating axis like the barbs of a 

 feather on their stalk ; and the secondary and parenchymatous, implanted on the 

 pulmonary walls of the first, like the hairs of a brush on their common base. 

 These two arrangements are evidently similar, except that the peripheral canals, 

 which are few, only form a single row on each side ; while the canaliculi, very 

 numerous, form several. Consequently, it may be said that the mode of ramifica- 

 tion proper to Mammalia is essentially dichotomous, and that observed in birds 

 essentially penniform. 



" Independently of the canaliculi arising from the pulmonary walls of the 

 diaphragmatic and costal bronchi, there are others which spring directly from the 

 generative trunk ; but in their dimensions, direction, form, and general disposi- 

 tion, they do not differ from the preceding. 



" How do these canals terminate ? Notwithstanding the importance of this 

 question, it has been generally neglected ; though its solution alone may furnish 

 the analogies and differences necessary for the parallel which has always been 

 attempted to be established between the lungs of birds and those of other verte- 

 brates. Our special researches on this point have led us to the conclusion that 

 all the canaliculi open into one another, and by this anastomosis constitute an 

 extricable plexus the various parts of which communicate with each other." 



Finally, it may be mentioned that " the walls of the pulmonary canaliculi 

 examined microscopically, appear to be covered internally with irregular septa 

 which circumscribe the areolte, and give them a cellular aspect." 



The Air-sacs.^ — " In birds, the pulmonary mucous membrane is continued, 

 at the level of the orifices in the lung, into the utriculiform cavities which are 

 developed between the walls of the thorax and the abdomen on the one side, and 

 the thoracic and abdominal viscera on the other. These air-reservoirs exist in 

 all the vertebrata of the second class. In all, they are situated at the periphery 

 of the viscera in the trunk, in such a manner that Cams has justly observed that 

 the lungs of Birds enclose all the other viscera ; so that when they are distended 

 by the entrance of air, they generally depress these viscera by pushing them 



' What is said relating to these air-sacs is taken from the Memoir of M. Sappey, Recherche* 

 eur I'Appareil Respiratoire des Oiseaux. Paris : 1847. 



I 



