558 THE RESPIRATORY APPAKAIUS IN BIRDS. 



convex and concave ; two borders — an external and an internal ; and two 

 extremities — an anterior and posterior. 



" The convex face, also named the dorsal, costal, or superior face, corresponds 

 inwardly to the dorsal vertebrae, and outwardly to the ribs and the intercostal 

 muscles ; it is exactly moulded on the walls of the thorax, and as the ribs pro- 

 trude on the internal face of these walls, it results that this surface of the lungs 

 is marked by transverse furrows which give it a lobulated aspect ; but these lobes 

 or lobules show nothing common with those composing the same organ in 

 Mammalia. In that class, the existence of lobes and lobules is an established 

 fact, and is caused by the dichotomous division of the bronchi ; in birds, it is 

 only apparent, and depends on the diminished thickness of the lung at each rib. 

 This face, quite imperforate, is covered by a thin layer of cellular tissue, which 

 unites it to the sides of the thorax. 



" The plane or concave face looks downwards ; it is in relation with the 

 diaphragm, which separates it from the viscera of the thorax, and from the 

 abdomen ; from this arises its other names of inferior, diaphragmatic, or visceral 

 face. Like the preceding, it is covered by a very fine layer of cellular tissue, 

 which forms adhesions with the diaphragm ; but it differs from it by the orifices 

 it presents, which are five in number, and constitute veritable canals, through 

 which the air passes and repasses incessantly to and from the sacs, and from them 

 to the lungs. 



" The borders are parallel to the axis of the body ; the internal is rectilinear, 

 thick, and rounded ; the external, convex, thin, and sharp. 



" Of the two extremities, the anterior, which is very acute, occupies the 

 receding angle formed by the spine within the first rib without ; the posterior, 

 more considerable, has a rounded form." 



In regard to structure, that which distinguishes the lung of Birds from that 

 of Mammals, is the mode of distribution and termination of the air-passages. In 

 Mammals, the large bronchial tubes, placed in the centre of the lung, send their 

 divisions towards the surface of the organ, or in a centrifugal manner ; in birds 

 they are disposed at the periphery of the lung, and direct their different ramifi- 

 cations towards the centre, or in a centripetal fashion. On the other hand, the 

 arboreal division of the bronchi in Mammals is replaced in birds by penniform 

 ramification. liastly, the terminal bronchial tubes instead of opening into a 

 series of closed vesicles, as in Mammals, anastomose with one another in birds, 

 so as to form an inextricable serial network. 



Sappey has further developed the knowledge acquired on this interesting 

 subject in the following terms : " Arrived in the pulmonary tissue, it (the 

 bronchial trunk) dilates, divides, gradually contracts in following its primary 

 direction, and in this way gains the posterior extremity of the organ, where it 

 terminates by opening into the abdominal reservoir. 



" This aerifying trunk, therefore, presents two very distinct portions, the one 

 extra-pulmonary, the other intra-pulmonary. The first offers the greatest analogy 

 to the bronchi of Mammalia ; it is membranous internally, elastic and fibrous 

 elsewhere, provided outwardly with cartilaginous rings, whicli embrace three- 

 fourths of its circumference, and is lined by mucous membrane characterized by 

 its pale rose-colour, and its marked adherence. 



" The second differs from the preceding in its dimensions, form, and struc- 

 ture. Owing to its dilatation at its entrance to the lung, its dimensions are more 

 considerable, and may be stated as three to two of the extra-pulmonary portion. 



