382 SMITHSONIAN MISCELLANEOUS COLLECTIONS l>OL. 5° 



The Diverticulum Axillare 



From the outer side of each lateral chamber of the interclavicular 

 sac processes arise which extend between the musculus coraco- 

 brachialis posterior and the musculus subcoracoideus, and usually 

 also between the musculus coraco-brachialis brevis and the distal 

 edge of the coracoid. These processes unite to form a spacious 

 chamber, the diverticulum axillare (fig. 11, DA 1). The chamber is 

 more or less completely subdivided into parts, and it has many out- 

 growths. The connection between its parts is slight, and in. many 

 other birds still slighter than in the pigeon. Most authors for this 

 reason distinguish here two such parts, the axillary proper and the 

 subpectoral, as distinct diverticula. The last named is an extension 

 of the axillary diverticulum and lies between the musculus coraco- 

 brachialis brevis and the coracoid. From it a distal outgrowth arises 

 which passes for some distance between the musculus supracoracoi- 

 deus and the musculus pectoralis major, and is always connected with 

 the diverticulum axillare proper. It does not attain any very consider- 

 able size. I do not regard this outgrowth as a true diverticulum ; 

 it is not very important, and in two cases was absent altogether. 



The principal part of the axillary diverticulum (figs. 2, 11, and 12, 

 DSSC 3) occupies the axillary space between the supracoracoid, sub- 

 coracoid, posterior coraco-brachial, great pectoral, and biceps mus- 

 cles. Medially where it abuts on the subcoracoid and supracora- 

 coid muscles, the axillary vessels and nerves partly pass along its 

 wall and partly penetrate it. The musculus pectoralis major limits 

 the space occupied by this diverticulum on the medial side. Dorsally 

 it extends to the ventral edge of the musculus scapuli humeralis pos- 

 terior. The part of the membranous wall of the air-sac that divides 

 it from the musculus pectoralis major is free ; all the rest is com- 

 pletely adherent to the adjacent muscles. With the exception of a 

 few insignificant fibers in the free membrane, I have found no proper 

 muscular bundles in the wall of this air-sac. 



From this diverticulum an outgrowth arises which passes over the 

 short portion of the musculus triceps cubiti (pars humero-cubitalis) 

 and enters- the humerus through the foramen pneumaticum of that 

 bone. The pars humero-cubitale of the musculus triceps cubiti arises 

 from the tuberculum minus humeri and the lower part of the crista 

 humeri. Together with the musculus scapuli humeralis posterior it 

 must alternately close and open the foramen during the beating of 

 the wing. I do not think that this has any great physiological 

 significance. More important must be the rhythmic change of volume 

 which the entire diverticulum undergoes during flight. From each 



