AVES. 



343 



immediately anterior to the great trochanter, 

 except in those Birds in which the femur 

 retains its medulla. 



The cervical air-cells are continued from the 

 large clavicular cell, and form in the Argala a 

 singular appendage or pouch, contained in a 

 loose fold of integument, which the bird can 

 inflate at pleasure. 



In the Pelecan 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 pas- 

 sage to the skin. The large pectoral muscles 

 and those of the thigh present a singular ap- 

 pearance, 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 cavi- 

 ties. Numerous strips of panniculus 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 

 inter-clavicular or furcular air-cell. The use 

 of these muscles appears to be to produce a 

 rapid collapse of the superficial air-cells, and 

 an expulsion of the air, when the bird is about 

 to descend, 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 different birds. In 

 the Raptores they are principally attached pos- 

 teriorly to the ribs, the diaphragmatic aponeu- 

 rosis 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 Mr. Hunter 

 injected the medullary cavities of the bones 

 from the trachea. It is stated that if the femur 

 into which the air is admitted be broken, the 

 bird shall not be able to raise itself in flight. 

 It is certain that if the trachea be tied, and an 

 opening be made into the humerus, the bird 

 will respire by that opening for a short period, 

 and may be killed by inhaling noxious gases 

 through it.* If an air-bone of a living bird, 

 similarly perforated, be held in water, bubbles 

 will rise from it, and a motion of the contained 



" I cut the wing through the os humeri in a 

 Fowl, and tying up the trachea found that the air 

 passed to and from the lungs by the canal in this 

 bone. The same experiment was made with the 

 os femoris of a young Hawk, and was attended with 

 a similar result. But the passage of air through 

 the divided parts, in both these experiments, espe- 

 cially in the last, was attended with more difficulty 

 than in the former one ; it was indeed so great, as 

 to render it impossible for the animal to live longer 

 than evidently to prove that it breathed through 

 the cut bone." Hunter's Animal (Economy, p. 94. 



air will be exhibited, synchronous with the 

 motions of inspiration and expiration. 



The proportion in which the skeleton is 

 permeated by air varies in different Birds. In 

 the Penguin ( Aptenodytes), which we have 

 examined for this purpose, air is not admitted 

 into any of the bones. Its chief progression being 

 in water, the specific levity of the body gained 

 by the substitution of air for marrow would be 

 rather a detriment than an advantage. The 

 condition of the osseous system, therefore, which 

 all birds present at the early periods of exist- 

 ence, is here retained through life. 



In the large Struthious Birds, which are re- 

 markable for the rapidity of their course, the 

 thigh-bones and bones of the pelvis, the ver- 

 tebral column, ribs, sternum and scapular 

 arch, the cranium and lower jaw, have all air 

 admitted into their cavities or cancellous struc- 

 ture. The humeri and other bones of the 

 wings, the tibiae and distal bones of the legs, 

 retain their marrow. 



With the exception of the Woodcock, all 

 Birds of Flight have air admitted to the 

 humerus. 



The Pigeon tribe, with the exception of the 

 Crown Pigeon, have no air in the femur, which 

 retains its marrow. In the Owls also the femur 

 is filled with marrow ; but in the Diurnal Birds 

 of Prey, as in almost all other Birds of Flight, 

 the femur is filled with air. 



In the Pelecan and Gannet the air enters all 

 the bones with the exception of the phalanges 

 of the toes. In the Hornbill even these are 

 permeated by air. 



Mr. Hunter* has given the following cha- 

 racters as distinguishing the bones which receive 

 air. They may be known " first, by their 

 less specific gravity ; secondly, by their retain- 

 ing little or no oil, and, consequently, being 

 more easily cleaned, and when cleaned, ap- 

 pearing much whiter than common bones : 

 thirdly, by having no marrow, or even any 

 bloody pulpy substance in their cells ; fourthly, 

 by not being in general so hard and firm as 

 other bones; and, fifthly, by the passage that 

 allows the air to enter the bones, which can 

 easily be perceived." 



We have reserved for this section the de- 

 scription of the foramina by which the air 

 penetrates the different bones. These openings 

 may be readily distinguished in the recent 

 bone, since they are not filled up by blood- 

 vessels or nerves, but have their external edges 

 rounded off. 



In the dorsal vertebrae the air-orifices are 

 small, numerous, and irregular ; situated along 

 the sides of the bodies, and the roots of the 

 spinous processes, the air passes into them 

 directly from the lungs. In the two or three 

 lower cervical vertebrae the air-holes are in the 

 same situation, but receive the air from the 

 lower cervical or clavicular air-cells : in the 

 remainder of these vertebrae the air-holes are 

 situated within the canal lodging the vertebral 

 artery, and communicate with the lateral air- 

 cells of the neck. 



* Animal (Economy, p. 91. 



