410 THE PIGEON. PHYLUM CHORDATA 



trachea is dilated and forms, with the beginnings of the bronchi, 

 the syrinx or organ of voice. Sound is produced by the vibration 

 of the membrana semilunaris, a delicate vertical fold of mucous 

 membrane, extending forwards from the angle between the 

 bronchi. The main trunks of the bronchi run right through the 

 lungs, which lie against the dorsal walls of the thorax covered 

 with peritoneum below only, into a series of large air sacs. These 

 are membranous cavities, seen when the sternum is removed in 

 dissection (Fig. 323). The median interclavicular sac, and some 

 of the others, send extensions into the bones. Inspiration is a 

 passive fall of the sternum, which draws air into the sacs ; in 

 expiration the sternum is raised, and the air is forcibly expelled 

 through recurrent bronchi into a network of air capillaries in 

 the lungs, and so by the main trunks of the bronchi into the 

 exterior. Expiration is thus the active phase, and it is during 

 this that the chief gas exchange takes place (Fig. 324). 



BLOOD VESSELS 



The body temperature is 42° C, which is higher than that of 

 mammals. This fact is no doubt connected with the active life 

 of the bird and the rapid metabolism which it necessitates. We 

 have already seen how the respiratory organs provide the ample 

 supply of oxygen which such metabolism demands. The red 

 corpuscles are oval and nucleated. The heart has four chambers, 

 two auricles and two ventricles, there being no sinus venosus 

 or conus arteriosus. The deoxygenated blood returned by the 

 venae cavae to the right auricle passes into the right ventricle 

 through an opening guarded by a muscular valve without chordae 

 tendineae. It is then driven by the pulmonary artery to the lungs, 

 whence it returns by the pulmonary veins to the left auricle, 

 passing thence through two membranous valves with chordae 

 tendineae to the left ventricle, by which it is driven into the single 

 aortic arch. The openings of the aorta and pulmonary artery are 

 guarded each by three semilunar valves. The aortic arch bends 

 over to the right side, giving off at its apex right and left in- 

 nominate arteries, from each of which arise a carotid and a sub- 

 clavian. The latter is not a true subclavian, being an expansion 

 of a segmental artery, and is exceedingly short, breaking up 

 immediately into brachial and pectoral branches. The further 

 course of the arteries is shown in Fig. 325. The venous system is 



