476 TEE RESPIRATORY APPARATUS. 



their likeness would be produced ; to obtain the f-ame results with the lungs of a reptile, ifc 

 is necessary to divide them in the direction of their great axis. 



" This shape enables us to distinguish in the lungs of a bird, two faces a convex 

 and concave ; two borders an external and internal -, and two extremities an anterior 

 and posterior. 



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

 to the dorsal vertebrse, and outwardly to the ribs arid the intercostal muscles ; it is 

 exactly moulded on the walls of the thorax, and as the ribs protrude on the internal face 

 uf these walls, it results that this surface of the lungs is marked by transverse furrows 

 which give it a lobulated aspect ; but these lobts 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 tho 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 and the first rib without ; the posterior, more consider- 

 able, 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 di if. rent ramifications 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. Lastly, 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. 



M. 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 iuto the 

 abdominal reservoir. 



"This serifying 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, which embrace three-fourths of its circum- 

 ference, and is lined by mucous membrane characterised by its pale rose-colour, and its 

 marked adherence. 



" The second differs from the preceding in its dimensions, form, and structure. 

 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. Beyond this enlarge- 

 ment, it diminishes in capacity by the emission of branches, losing its cylindrical form 

 to assume that of a cone with a truncated 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 framework of 

 the lung, are remarkable for their uniformity in numbi r, form, and the direction they 

 offer in all classes of birds. They are generally twelve, and their origin is thus distri- 

 buted : 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 



