344 



AVES. 



The air-holes of the vertebral ribs are situated 

 at the internal surface of their vertebral extre- 

 mities, and appear like tnose of the contiguous 

 vertebrae to have an immediate communication 

 with the lungs. The sternal ribs, or ossified costal 

 cartilages, have also internal cavities which 

 receive air from the lateral thoracic cells by 

 means of orifices placed at their sternal ex- 

 tremities. 



The orifices by which air is admitted to the 

 sternum are exceedingly numerous, but are 

 principally situated along the mesial line of the 

 internal surface, opposite the origin of the 

 keel, forming a reticulation at that part; the 

 largest foramen is near the anterior part of 

 the bone ; some smaller ones occur at the 

 costal margins. All these orifices commu- 

 nicate with the thoracic air-receptacles. 



The scapula is perforated by several holes 

 at the articular extremity, which admit air 

 into its cancellous structure from the axillary 

 cell. 



The coracoid has small air-holes at both ex- 

 tremities ; the largest is situated on its inner 

 surface, where it is connected with the clavicle 

 or furculum. 



The furculum receives air principally by a 

 small hole in the inner side of each of its 

 scapular extremities, which communicates with 

 the clavicular air-cell- 



The air-hole of the humerus is of large size, 

 and situated at the back part of the head of 

 the bone, below the curved inferior process. 

 It communicates with the axillary air-cell, and 

 transmits the air to the cavity of the bone by 

 several cribriform foramina. 



The air-holes of the pelvic bones are situated 

 irregularly on the inner surface upon which the 

 kidneys rest, and must therefore receive air 

 from continuations of the abdominal receptacles 

 around the kidneys. 



The air-hole, or rather air-depression of the 

 femur, is situated at the anterior part of the 

 base of the trochanter; it receives air from the 

 glutseal cell, and transmits it by several small 

 foramina into the interior of the bone. In 

 the Ostrich, the air-holes are situated at the 

 posterior part of the bone at both of its extre- 

 mities. 



The cavities of the long bones into which 

 air is thus admitted are proportionally larger 

 than in the corresponding bones of Mammalia, 

 and are characterized by small transverse 

 osseous columns which cross in different di- 

 rections from side to side, and are more nu- 

 merous near the extremities of the bone ; they 

 abut against and strengthen, like cross-beams, 

 the parietes of the bone. 



We have sometimes succeeded in filling with 

 fine size-injection the minute arteries which 

 ramify on the membrane lining these cavities, 

 but the vascularity of this membrane is by no 

 means very remarkable. 



The lower jaw receives its air by means of 

 an orifice situated upon each ramus behind the 

 tympano-maxillary articulation. Mr. Hunter 

 was in doubt as to whether the lower jaw 

 derived its supply of air from the Eustachian 

 tube or the trachea where it passes along the 



neck.* In a Pelecan which we dissected for 

 the purpose we found it to be supplied by an 

 air-cell which surrounded the joint, and was con- 

 tinuous with the upper cervical air-cells. The 

 bones of the cranium and upper jaw have com- 

 munications with the Eustachian tube, but not 

 with the nasal passages, which are every where 

 lined with an impervious pituitary membrane. 



Various explanations have been given of the 

 final intention of the condition of the respiratory 

 system above described. 



The extension of this system by means of 

 continuous air-receptacles throughout the body 

 is subservient to the function of respiration, 

 not only by a change in the blood of the 

 pulmonary circulation effected by the air of the 

 cells on its re-passage through the bronchial 

 tubes, but also, and more especially, by the 

 change which the blood undergoes in the ca- 

 pillaries of the systemic circulation, which are 

 in contact with the air-receptacles. The free 

 outlet to the air by the bronchial tubes does 

 not, therefore, afford an argument against the 

 use of the air-cells as subsidiary respiratory 

 organs, but rather supports that opinion, since 

 the inlet of atmospheric oxygenated air to be 

 diffused over the body must be equally free. 



A second use may be ascribed to the air- 

 cells as aiding mechanically the actions of 

 respiration in Birds. During the act of inspi- 

 ration the sternum is depressed, the angle 

 between the vertebral and sternal ribs made 

 less acute, and the thoracic cavity proportion- 

 ally enlarged ; the air then rushes into the 

 lungs and into the thoracic receptacles, while 

 those of the abdomen become flaccid : when 

 the sternum is raised or approximated towards 

 the spine, part of the air is expelled from the 

 lungs and thoracic cells by the trachea, and 

 part driven into the abdominal receptacles, 

 which are thus alternately enlarged and dimi- 

 nished with those of the thorax. Hence the 

 lungs, notwithstanding their fixed condition, 

 are subject to due compression through the 

 medium of the contiguous air-receptacles, and 

 are affected equally and regularly by every 

 motion of the sternum and ribs. 



A third use, and perhaps the one which is 

 most closely related to the peculiar exigences 

 of the bird, is that of rendering the whole 

 body specifically lighter ; this must necessarily 

 follow from the dessication of the marrow and 

 other fluids in those spaces which are occupied 

 by the air-cells, and by the rarefaction of the 

 contained air from the heat of the body. 



Agreeably to this view of the function of the 

 air-cells, it is found that the quantity of air 

 admitted into the system is in proportion to the 

 rapidity and continuance of the bird's motion; 

 and that the air is especially distributed to those 

 members which are most employed in loco- 

 motion ; thus the air is admitted into the wing- 

 bones of the Owl, but not into the femur; 

 while in the Ostrich the air penetrates the 

 femur, but not the humerus or other bones of 

 the wing. 



A fourth use of the air-receptacles, which 

 has not hitherto been suspected, relates to the 



* Loc. cit. p. 93. 



