344 AVES. 



The air-holes of the vertebral ribs are situated neck.* In a Pelecan which we dissected for 

 at the internal surface of their vertebral extre- the purpose we found it to be supplied by an 

 mities, and appear like tnose of the contiguous air-cell which surrounded the joint, and was con- 

 vertebrae to have an immediate communication tinuous with the upper cervical air-cells. The 

 with the lungs. The sternal ribs, or ossified costal bones of the cranium and upper jaw have corn- 

 cartilages, have also internal cavities which munications with the Eustachian tube, but not 

 receive air from the lateral thoracic cells by with the nasal passages, which are every where 

 means of orifices placed at their sternal ex- lined with an impervious pituitary membrane, 

 tremities. Various explanations have been given of the 



The orifices by which air is admitted to the final intention of the condition of the respiratory 



sternum are exceedingly numerous, but are system above described. 



principally situated along the mesial line of the The extension of this system by means of 



internal surface, opposite the origin of the continuous air-receptacles throughout the body 



keel, forming a reticulation at that part; the is subservient to the function of respiration, 



largest foramen is near the anterior part of not only by a change in the blood of the 



the bone ; some smaller ones occur at the pulmonary circulation effected by the air of the 



costal margins. All these orifices commu- cells on its re-passage through the bronchial 



nicate with the thoracic air-receptacles. tubes, but also,, and more especially, by the 



The scapula is perforated by several holes change which the blood undergoes in the ca- 



at the articular extremity, which admit air pillaries of the systemic circulation, which are 



into its cancellous structure from the axillary in contact with the air-receptacles. The free 



cell. outlet to the air by the bronchial tubes does 



The coracoid has small air-holes at both ex- not, therefore, afford an argument against the 



tremities ; the largest is situated on its inner use of the air-cells as subsidiary respiratory 



surface, where it is connected with the clavicle organs, but rather supports that opinion, since 



or furculum. the inlet of atmospheric oxygenated air to be 



The furculum receives air principally by a diffused over the body must be equally free, 

 small hole in the inner side of each of its A second use may be ascribed to the air- 

 scapular extremities, which communicates with cells as aiding mechanically the actions of 

 the clavicular air-celU respiration in Birds. During the act of inspi- 



The air-hole of the humerus is of large size, ration the sternum is depressed, the angle 

 and situated at the back part of the head of between the vertebral and sternal ribs made 

 the bone, below the curved inferior process, less acute, and the thoracic cavity proportion- 

 It communicates with the axillary air-cell, and ally enlarged ; the air then rushes into the 

 transmits the air to the cavity of the bone by lungs and into the thoracic receptacles, while 

 several cribriform foramina. those of the abdomen become flaccid : when 



The air-holes of the pelvic bones are situated the sternum is raised or approximated towards 



irregularly on the inner surface upon which the the spine, part of the air is expelled from the 



kidneys rest, and must therefore receive air lungs and thoracic cells by the trachea, and 



from continuations of the abdominal receptacles part driven into the abdominal receptacles, 



around the kidneys. which are thus alternately enlarged and dimi- 



The air-hole, or rather air-depression of the nished with those of the thorax. Hence the 



femur, is situated at the anterior part of the lungs, notwithstanding their fixed condition, 



base of the trochanter; it receives air from the are subject to due compression through the 



glutaeal cell, and transmits it by several small medium of the contiguous air-receptacles, and 



foramina into the interior of the bone. In are affected equally and regularly by every 



the Ostrich, the air-holes are situated at the motion of the sternum and ribs, 

 posterior part of the bone at both of its extre- A third use, and perhaps the one which is 



mities. most closely related to the peculiar exigences 



The cavities of the long bones into which of the bird, is that of rendering the whole 



air is thus admitted are proportionally larger body specifically lighter ; this must necessarily 



than in the corresponding bones of Mammalia, follow from the dessication of the marrow and 



and are characterized by small transverse other fluids in those spaces which are occupied 



osseous columns which cross in different di- by the air-cells, and by the rarefaction of the 



rections from side to side, and are more nu- contained air from the heat of the body, 

 merous near the extremities of the bone ; they Agreeably to this view of the function of the 



abut against and strengthen, like cross-beams, air-cells, it is found that the quantity of air 



the parietes of the bone. admitted into the system is in proportion to the 



We have sometimes succeeded in filling with rapidity and continuance of the bird's motion; 

 fine size-injection the minute arteries which and that the air is especially distributed to those 

 ramify on the membrane lining these cavities, members which are most employed in loco- 

 but the vascularity of this membrane is by no motion ; thus the air is admitted into the wing- 

 means very remarkable. bones of the Owl, but not into the femur ; 



The lower jaw receives its air by means of while in the Ostrich the air penetrates the 



an orifice situated upon each ramus behind the femur, but not the humerus or other bones of 



tympano-maxillary articulation. Mr. Hunter the wing. 



was in doubt as to whether the lower jaw A fourth use of the air-receptacles, which 



derived its supply of air from the Eustachian has not hitherto been suspected, relates to the 

 tube or the trachea where it passes along the * Loc. cit. p. 93. 



