MUI.LER] THE AIR-SACS OF THE PIGEON 369 



ner quite different from that of the mammalian diaphragm ; and the 

 abdominal diaphragm, which separates the air-sacs from the intestine 

 and has nothing to do with respiration. 



The Pulmonary Diaphragm (Fig. i, DIP) 



(Anterior, transverse or trilateral diaphragm; Pulmonary aponeurosis, Hux- 

 ley ; Diaphragme pulmonaire, Sappey ; Diaphragmite anterieur, Milne 

 Edwards.) 



This diaphragm is formed by that portion of the pleura which 

 covers, as an aponeurotic septum, the ventral surface of each lung. 

 The medio-ventral surfaces of the two lungs closely adhere to the 

 diaphragm, which is penetrated by blood-vessels and by the tubes 

 connecting the lungs with the air-sacs. Where the two halves of 

 the pulmonary diaphragm approach each other in the median plane, 

 they adhere to the mediastinum which extends in the shape of a 

 vertical septum in the median plane of the body, is attached to the 

 dorsal vertebrae, and separates the two lungs. This diaphragm, 

 therefore, shuts off from the rest of the body-cavity a special space, 

 the Cavum pulmonale, which contains the lungs and the small intra- 

 thoracic portion of the cervical sacs. 



Being adherent to the pleura, this membrane assists in the 

 expansion of the lungs during inspiration. It is enabled to per- 

 form this function by the two pulmonary muscles of Perrault (fig. 

 8, MP), which in many birds are strongly developed. In the 

 pigeon these muscles are not so large as in some other birds, as, 

 for example, the cassowary, but they are not at all rudimentary 

 and are certainly functional. These muscles arise from the true 

 ribs near their sterno-costal articulations, and run, parallel to the 

 ribs, obliquely downward to the pulmonary diaphragm. When they 

 contract, this diaphragm is drawn downward and becomes ventrallv 

 convex, the pulmonary cavity is enlarged, and the lungs expand. 

 The muscles arising from the anterior ribs are membranous, and 

 unite to form a muscular sheet which, although quite thin in some 

 places, is nowhere pierced by an air-tube. The muscle arising from 

 the lowest rib is more cylindrical in form and not so completely 

 coalesced with those in front as they are with each other. Its inser- 

 tion coincides with the partition that separates the anterior and 

 posterior sacci intermedii, and extends as far as the proximal edge 

 of the ostium intermedium posterius. From the last rib a second 

 flat pulmonary muscle arises medially, which appears as a branch 

 of the one just mentioned. This muscle extends between the ostium 

 intermedium posterius (figs. 8, 9, 10, 12, OIP) and the ostium pos- 

 terius (figs. 8, 9, 10, 12, OP), and is attached to the diaphragm. 

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