EESPTEATORY SYSTEM OF BIEDS. 213 



Both the abdominal receptacles transmit air to the pelvic cells, 

 ^, li, of their respective sides, and to several small and extremely 

 delicate cells between and behind the coils of intestine. One of 

 these is continued round the fold of the duodenum and pancreas 

 to the gizzard, and has been termed the duodenal cell. 



From the inguinal cell are continued the intermuscular cjlutceal 

 and femoral cells, which surround the head of the femur, and 

 communicate with that bone by an aperture, /, situated imme- 

 diately anterior to the great trochanter, except in those Birds in 

 which the femur retains its medulla. 



The cervical air-cells are continued from the laro-e clavicular 



o 



cell, and form in the Argala and Bustard, fig. 54, a, a singular 

 appendage or pouch, contained in a loose fold of integument, 

 which the bird can inflate at pleasure. 



In the Pelican and Gannet extensive air-cells are situated 

 beneath almost the whole of the integument of the body, which is 

 united to the subjacent muscles only here and there by the septa 

 of the cells and the vessels and nerves which are supported by the 

 septa in their passage to the skin. The large pectoral muscles and 

 those of the thigh present a singular appearance, being, as it were, 

 cleanly dissected on every side, having the air-cavities above and 

 beneath them. The axillary vessels and nerves are also seen 

 passing bare and unsupported by any surrounding substance 

 through these cavities. Numerous strips of j)anniculus carnosus 

 pass from various parts of the surface of the muscles to be firmly 

 attached to the skin ; a beautiful fan-shaped muscle is spread over 

 the interclavicular or furcular air-cell. The use of these muscles 

 appears to be to j^roduce a rapid collapse of the superficial air- 

 cells, and an expulsion of the air, when the bird is about to de- 

 scend, in order to increase its specific gravity, and enable it to 

 dart with rapidity upon a living prey. 



The air-receptacles of the thoracic-abdominal cavity present 

 varieties in their relative sizes and modes of attachment in dif- 

 ferent birds. In the Raptores they are principally attached pos- 

 teriorly to the ribs, the diaphragmatic aponeurosis covering the 

 lungs, and to the kidneys; while in the Grallatores they have 

 anterior attachments to the intestines in many places. 



The singular extension of the respiratory into the osseous 

 system was discovered almost simultaneously by Hunter and 

 Camper, and ably investigated by them through the whole class 

 of Birds. The air-cells and lungs can be inflated from the bones, 

 and Hunter injected the medullary cavities of the bones from the 

 trachea. If the femur into which the air is admitted be broken. 



