216 ANATOi\IY OF VERTEBRATES. 



numerous near the extremities of the bone ; they abut against and 

 strengthen, like cross-beams, the parietes of the bone. The 

 membrane lining these cavities, is not very vascular. 



The lower jaw receives its air by an orifice situated upon each 

 ramus behind the tympanic articulation, from an air-cell which 

 surrounds the joint. The bones of the cranium and upper jaw 

 receive air admitted to the tympanic cavity by the Eustachian 

 tube, not from the nasal passages. With these, however, the 

 subocular air-cell communicates ; and in the Coot, Water-hen, 

 Goose, and other water-birds, entozoa (^Monostoma mutahile^ e. g.) 

 gain access to that air-cell. 



The extension from the lungs 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 receptacles on its repassage through the 

 bronchial tubes ; but also, and more especially, by the change 

 which the blood undergoes in the capillaries of the systemic cir- 

 culation, 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 respi- 

 ratory 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 me- 

 chanically the actions of respiration in Birds. During the act of 

 inspiration the sternum is depressed, the angle between the 

 vertebral and sternal ribs made less acute, and the thoracic cavity 

 proportionally enlarged ; the air then rushes into the lungs and 

 into the thoracic receptacles, wdiile 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 j^art driven into the abdominal receptacles, 

 which are thus alternately enlarged and diminished with those of 

 the thorax. Hence the luncfs, notwithstandino' their fixed con- 

 dition, 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 Y\hs. 



A third use, and perhaps the one which is most closely related 

 to the peculiar exigencies of the bird, is that of rendering the whole 

 body specifically lighter; this must necessarily follow from the 

 desiccation of the marrow and other fluids in those spaces which 

 are occupied by the air-cells, and by the rarefaction of the con- 

 tained air from the heat of the bod v. 



