MULLEE] THE AIR-SACS OF THE PIGEON 389 



three small chambers covered for the greater part by the mus- 

 culus ilio-fibularis. This diverticulum femorale posterius and 

 the diverticulum femorale anterius, described above, form the in- 

 guinal diverticulum of Owen. I have never observed the divertic- 

 ulum that Baer describes (1896, p. 440) as passing out through the 

 foramen obturatorium. As Baer also mentions nerves and vessels 

 coming out through this foramen, it appears probable that there is 

 here some confusion of nomenclature. Considering the lack of 

 precision in much of the anatomical data concerning birds, such a 

 confusion is likely enough to arise. 



The inguinal diverticula surround the acetabulum on all sides, so 

 that the head of the femur rests upon a ring of air-cushions. This 

 arrangement doubtless increases the mobility of the femur and re- 

 duces the friction of the adjacent muscles. 



The uppermost chamber of the inguinal system, the diverticulum 

 femorale superius (figs. 6, 11 and 12, DFS), is covered only by the 

 flat tendon of the musculus ilio-femoralis externus and the skin, so 

 that its situation is similar to that of the superficial portion of the 

 abdominal sac above mentioned (fig. 6, SAD, SAS). A penetration 

 of this diverticulum, which is a branch of the prsefemoral divertic- 

 ulum, does not, however, give as satisfactory physiological results 

 as a penetration of the wall of the abdominal sac, because the prge- 

 femoral diverticulum communicates with the other parts of the air- 

 sac system only by very devious and narrow passages which are 

 easily closed by the contraction of the contiguous muscles. 



General Considerations Concerning the Air-sacs of the 

 Pigeon that Lie Between the Soft Parts. 



While the air-sacs of the pigeon show a great diversity in detail 

 and are in some regions subject to very considerable variations, they 

 are, in their general features, constant enough to enable one to give an 

 account of their typical form. I have not regarded in the foregoing 

 descriptions the numerous pathological divergences from the normal 

 type met with in the investigation. The pathological changes and 

 other variations in size that affect the liver and other organs have a 

 marked influence upon the air-sacs. The deposits of fat found so 

 frequently about the pericardium and elsewhere, in spaces that would 

 otherwise be occupied by air-sacs, also play an important part in this 

 respect. 



Subcutaneous air-spaces, such as occur in the pelican and some 

 other birds, are entirely absent in. the pigeon. I have, it is true, 



