384 SMITHSONIAN MISCELLANEOUS COLLECTIONS [vol. 50 



The sacci intermedii are two pairs of air-sacs that closely corre- 

 spond to each other in their configuration. This correspondence is 

 so marked that most authors have given expression to it in the 

 names applied to them. They occupy spaces morphologically 

 and functionally similar between the pulmonary diaphragm, the 

 abdominal diaphragm, and the outer wall of the thorax. These 

 sacs are simple in shape and have no diverticula or secondary com- 

 munications. The saccus intermedins of each side is divided into 

 two parts, an anterior, the saccus intermedius anterior, and a pos- 

 terior, the saccus intermedius posterior. 



The Sacci Intermedii Anteriores 



According to Bertelli (1900, p. 162), these sacs are visible, in the 

 developing chick, on the sixth day of incubation. On the tenth 

 day they appear as flat vesicles spreading over the concave, ventral 

 surfaces of the lungs. They arise from the right and left ostia in- 

 termedia anteriora. Once I saw several small ostia in the place of 

 the single one. The sacs themselves (figs. 1, 11, and 12, SIA) 

 occupy the anterior part of the space between the pulmonary and 

 abdominal diaphragms, and are separated from the interclavicular 

 sacs only by the air-sac walls themselves. They are apparently sim- 

 ilarly developed in all birds. The following description, given by 

 Roche (1891, p. 32) as that of their general type, applies also to the 

 pigeon : "Les reservoirs anterieurs se trouvent ainsi delimites etant 

 en rapport avec les poumons, en haut ; la premiere cote, et l'artere 

 pulmonaire, de chaque cote, en avant ; les cotes en bas ; le foie en 

 dehors, ainsi que le cceur et le diverticule precardiaque du sac 

 claviculaire ; enfin, les reservoirs posterieurs, en arriere." These 

 sacs are not in direct contact with the lungs, but separated from 

 them by the pulmonary diaphragm. The pulmonary muscles of 

 Perrault protrude into them. Each sac has the shape of a wedge, 

 its base resting on the pulmonary diaphragm and its edge directed 

 ventrally, and lying near the margin of the sternum. Backwards 

 each sac extends to the last rib. Medially its posterior wall bends 

 forward and unites with the pulmonary diaphragm. Since this 

 sac has no free wall — even its medial side adhering to the pulmon- 

 ary diaphragm — it does not collapse when the body cavity is opened, 

 and it is probable that also during life it does not undergo any 

 considerable changes in volume. It is, of course, strongly dilated 

 during inspiration ; since, however, the pulmonary muscles of Per- 

 rault undoubtedly draw the lung into the space it occupies this 

 change of volume (dilatation) must to a great extent be compensated. 



