480 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 52. 



depression in the dorsal wall of the trachea immediately behind the 

 larynx and lies between the trachea below and the esophagus above. 

 When fully distended this air-sac like those of the pulmonary system 

 is foimd to have thin transparent walls. In form it is somewhat pear- 

 shaped, flattened ventraUy and rounded dorsally. The anterior 

 end for a space of 8 or 10 mm. is narrow, forming a slender neck 

 which broadens abruptly into the body of the sac. In one specimen 

 (Cat. No. 224829, U.S.N.M.), the distended sac measured 50 mm. 

 long by 43 mm. broad. The depth was equal to the lateral measure- 

 ment (43 mm.). In another (Cat. No. 224831, U.S.N.M.) the vesicle 

 measured 65 mm. long by 32 mm. wide at the broad basal end. The 

 diameter of the anterior extension or neck of the sac in this bird was 

 10 mm. In a third bird (Cat. No. 224832, U.S.N.M.) the measure- 

 ments were mtermediate between these two. 



In dissecting these ruddy ducks I found that the paired dermal 

 muscles (called by Gadow^ the deeper layer of the muscle cucullaris) 

 arise on the side of the head posterior to the orbit and pass backward 

 converging immediately below the larynx until separated by only 

 the space of a milhmeter. The two muscles expand at once to cover 

 entirely the loose skin of the ventral sides of the neck. They insert 

 by two strong attachments on the furculum. For its anterior half 

 each muscle is a single broad sheet. Posteriorly, (covering the 

 tracheal air-sac) each divides into 20 or 22 fasciculae. While the 

 lateral bands of each muscle are thin and weak the series of fascicles 

 increases markedly in strength as the median hue is approached. 

 MacGilhvray, in his notes on the trachea in the male ruddy duck,^ 

 remarks that the tracheal muscles are similar to those of other ducks 

 but in this he was mistaken. The paired muscle stemo-trachealis is 

 inserted as a slender band on either side of the thyroid cartilage. 

 Passing back on reaching the anterior end of the tracheal air-sac it 

 broadens suddenly to a width of 10 mm. The anterior edge of the 

 expansion attaches fii-mly at its outer angle to the esophagus. Each 

 muscle narrows gradually imtil at the posterior margm of the air-sac 

 it is only 2 mm. wide. It is closely attached to the air-sac touch- 

 ing the trachea only along its mternal edge. Posteriorly the entire 

 narrowed muscle transfers rather abruptly from the air-sac to the 

 trachea. 



The depression marking the mouth of the air-sac is immediately 

 behmd the larynx on the dorsal surface of the trachea (fig. 1, d). 

 Its sides are abrupt and it is broad anteriorly, somewhat constricted 

 medially, and truncated posteriorly. From its posterior end leads 

 the canal for the passage of air. In one specimen examined four of 

 the anterior tracheal rings are interrupted over the opening of the 

 canal; in another these rings are entire. 



1 Gadow, Hans, Bronn's Klassen und Ordniingen des Thier-Reichs. Vogel, vol. 1, 1891, p. 215. 

 > Adubon, ,T. J., Ornithological Biography, vol. 4, 1838, p. 331. 



