. The structure and life of birds . r the intestines,extending far back behind the lungs. In frontof them are the posterior thoracic, and next tothem the anterior thoracic, sacks. Then comes theinterclavicular sack formed of two which have coa-lesced. The middle part of this can easily be seenin the angle between the clavicles or wishbones,but it also runs out on either side to the shoulderbones (Fig. 23). The cervical sacks are very smalland lie at the base of the neck. As yet I have only described the minimum of airsacks common to all birds : in many species there areair cavities in the bone
. The structure and life of birds . r the intestines,extending far back behind the lungs. In frontof them are the posterior thoracic, and next tothem the anterior thoracic, sacks. Then comes theinterclavicular sack formed of two which have coa-lesced. The middle part of this can easily be seenin the angle between the clavicles or wishbones,but it also runs out on either side to the shoulderbones (Fig. 23). The cervical sacks are very smalland lie at the base of the neck. As yet I have only described the minimum of airsacks common to all birds : in many species there areair cavities in the bones, sometimes extending even tothe very extremities of the limbs : in some they arefound under the skin also, and even in some of thefeathers and between the muscles. In a young bird G 82 THE STRUCTURE AND LIFE OF BIRDS CHAP. the bones are always filled with marrow, but often asit grows to maturity the marrow is absorbed, leavingonly a thin dry-looking lining, and the delicate mem-brane of the air-sacks extends into the cavity. Thus. Fig. 24.—Section of (a) femur of Ostrich ; (l>) skull of Carinate Bird. E, externalopening to ear. The bronchial membrane lines all the small cavities in the bones. whenever a bone is hollow (if we except certainparts of the skull), the cavity connects with the lungsand is lined with the bronchial membrane. When thecavity in the bone is large, thin plates separate from vi FORM AND FUNCTION g3 the inner coat and act as buttresses. Sometimes thesebuttresses are bound together and a strong network isformed. It is a network like this which supports thebeak. In the skull the plates take the form of all birds without exception, I believe, some of thebones of the skull are aerated, the air being derivedmainly from the nostrils and ears. But the beak andsome of the bones connected with it are aerated fromthe lungs. Thither runs from each cervical air-sack asmall tube of membrane which lies in an incompletebony canal under the vertebrae by the
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Keywords: ., bookcentury1800, bookdecade1890, bookidstructurelif, bookyear1895
