MILLER] THE AIR-SACS OF THE PIGEON 379 



Gouillot: Reservoir infralaryngien. 



Huxley: Infrabronchial air-sac. 



Jacqucmin: Poche pneumatique sous-claviculaire. 



Merrem: Bulla cordis anterior. 



Milne Edwards: Reservoir claviculaire. 



Owen: Anterior thoracic cell. 



Sappcy: Reservoir thoracique. 



Tiedemann: Vordere Herzluftzelle. 



In full-grown pigeons there is, as in most other birds when full 

 grown, onLy a single, unpaired, interclavicular sac. This arises from 

 the union in the median plane of two symmetrical, originally com- 

 pletely separate structures. The two distinct sacs which coalesce to 

 form the interclavicular cavity arise from the right and left ostia 

 clavicularia, respectively (figs. 9 and 12, OCL). 



In the chick, Bertelli (1900, p. 162) noted these two sacs as early 

 as the sixth day of incubation. They are at first situated dorsally 

 and laterally ; they afterwards grow downwards and towards the 

 median plane. They reach their definite position at the end of the 

 eighth day. These sacs give off diverticula which at the time of 

 hatching, have already reached the humerus. Their ingrowth into 

 this bone takes place later. The diverticula of these sacs are very 

 much larger and more complicated than the sacs themselves. As 

 Hunter (1774, p. 209) noted long ago, the median or central portion 

 of the sac is permeated by numerous trabecular. Hunter says con- 

 cerning them : "The superior part of the lungs opens into the large 

 cells of a loose network, through which the trachea, oesophagus, and 

 large vessels, going from, and coming to the head, pass." 



The interclavicular sac occupies the anterior part of the thoracic 

 cavity, and extends from the membrane stretched out between the 

 branches of the furculum (fig. 7, MF) to the base of the heart, the 

 sacci intermedii anteriores on both sides of the latter, the free ribs, 

 the coracoid, the ligamentum sterno-furculare, and the anterior por- 

 tion of the sternum. Dorsally the trachea, the oesophagus, the jugu- 

 lar veins, and the vagus nerves separate it from the cervical sacs. 

 The anterior end of the heart, the great vessels, the bifurcation of the 

 trachea, the two bronchi, and the oesophagus lie pretty freely in 

 the cavity of the interclavicular sac. The various parts of the in- 

 terclavicular sac occupy the space between these organs. Struc- 

 tures indicating the origin of the sac from two distinct points can not 

 be made out in the full-grown pigeon, but we observe that the whole 

 sac is partially divided into three chambers, a medial and two lateral 

 ones. 



