526 THE GANNET 



when on the wing, is admitted, but a Gannet when standing 

 evidently has httle or no power of inflation. 



The subcutaneous air-sacs develop very early. They 

 are present in a Gannet when only three weeks old, and 

 at the age of four months they are very large. 



Function of the Air-sacs. — Unquestionably this net-work 

 of inflatable cells under the skin helps to break the 

 force of the heavy plunge with which a Gannet drops 

 into the water, and it is reasonable to think that this 

 is the primary object for which the cells are intended. 

 This is the view taken by Macgillivray and also by 

 Mr. F. A. Lucas (" Natural Science," 1894, p. 36), yet 

 it may be they serve other purposes as well. There must 

 be a large amount of space between a Gannet's skin 

 and its body, when filled out with air for plunging. 

 As it penetrates the water, the skin is drawn in and the 

 air expelled through the lungs, probably leaving the cells 

 empty. 



But we have not yet finished with the air-receptacles 

 of this bird. Between the posterior portion of the pectoral 

 muscles and the ribs there is a large cavity, and another 

 between the anterior portion and the sternum itself, extend- 

 ing to the humerus. These internal cavities must be cap- 

 able of holding nearly as great an amount of air as those 

 cells which lie immediately under the skin. 



