MftlAEE] THE AIR-SACS OF THE PIGEON 393 



parts of the walls of the sacs, except those near the ostia, where the 

 inhaled dust is ejected by the cilia, are strongly impregnated with 

 soot. The condition of the atmosphere of Prague, in which lived 

 the pigeons examined, is very favorable to the development of this 

 phenomenon ! By treatment with nitrate of silver the boundaries 

 of the irregularly polygonal cells, of which the pavement epithelium 

 of the distal portions of the air-sacs is composed, are easily demon- 

 strated. According to Fourneux and Hermann (1876, p. 218), these 

 cells are similar to those of the epithelium of the posterior portion 

 of the lungs of snakes. A similar epithelium, composed, however, 

 of somewhat larger cells, clothes the walls of the sacs within the 

 cavities of the pneumatic bones. In some places there are found, 

 between these cells, groups of smaller ones, the true nature of which 

 is doubtful. Baer (1896, p. 450) regards them as growth-centers 

 for the pavement epithelium, and believes that the larger cells are 

 produced by a coalescence of such smaller ones. 



Externally the membrana propria of the air-sac wall is covered, 

 wherever it is free, by an endothelium whose cells are somewhat 

 smaller than those of the inner epithelium. In this external endo- 

 thelial layer lymph-stomata occur. 



Contrary to the accounts given by Lereboullet (1838), Leydig 

 (1857, p. 376), and Eberth (1863, p. 434), I have never been able to 

 find muscles in the proper walls of the air-sacs of pigeons ; at the 

 places where these authors thought they saw them I failed to sec 

 anything but bundles or strands of elastic fibers. Such occur in 

 the wall of the interclavicular sac near the furcula, in the wall of the 

 cervical sacs (cf. Sappey, 1874, p. 38), and elsewhere. In this re- 

 spect my results agree with Baer's (1896, p. 448). 



A good resume of the literature relating to the structure of the 

 walls of the air-sacs is found in Oppel ( 1905, p. 333). 



The Function of the Air-sacs 



Much has been written on the function of the air-sacs and the 

 advantages birds derive from their possession ; that, however, which 

 I hold to be their true nature and utility has either been overlooked 

 altogether or far underrated in importance. 



The most important hypotheses advanced concerning the function 

 of the air-sacs are the following: 



One hypothesis is that the air-sacs serve to erect the feathers. 

 Since, however, this could only be effected by subcutaneous sacs, 

 such as occur in the pelican, and since such sacs are wanting in 



