SEC. IV ANATOMY OF BIRDS 297 



much in extent find disposition as to be not easily described 

 except either in the most general terms already used, or with 

 particularity of detail for different species. According to Owen, 

 however, the usual disposition is : An interclavicular air-space, 

 quite constant : this, with its cervical prolongations, furnishes the 

 great " air-drums " of the pinnated grouse and cock-of-the-plains. 

 Anterior thoracic, about the roots of the lungs. Lateral thoracic, pro- 

 longed to axillary, and to spaces and passages in the wings, includ- 

 ing the hollow humerus. Large hepatic or posterior thoracic, about 

 the lower part of the lung and the liver. Abdominal, right and left, 

 of great size, from the lower part of the lung where the longest 

 bronchial tubes open very freely ; extending to pelvic and inguinal 

 compartments, whence femoral sacs, the hollow of the femur, etc. 

 The subcutaneous cells are enormously developed in the pelican and 

 gannet ; the extensive areolar tissue being thoroughly pneumatic, 

 and furnished with an arrangement of the cutaneous muscle (pan- 

 niculus carnosus) whereby^ apparently, the air may be rapidly and 

 forcibly expelled by compression. A similar muscle develops in 

 some birds in connection with the interclavicular air-space. (The 

 pneumaticity of the skeleton has been already treated.) 



The purpose of this extensive respiratory apparatus is thus 

 dwelt upon by the great English anatomist just cited : " 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 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 action of respiration in 

 birds. During the act of inspiration the sternum is depressed 

 [lowered from the back-bone in horizontal position of a bird], 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 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 through the trachea, and part driven into the abdominal 

 receptacles, which are thus alternately enlarged and diminished with 

 those of the thorax. Hence the lungs, notwithstanding their fixed 

 condition, are subject to due compression through the medium of 



