388 ZOOLOGY 



SECT. 



off secondary bronchi, and these branch again, sending off tubes 

 which give rise to a system of fine branching and anastomosing 

 tubules, the " lung-capillaries," which make up the main substance 

 of the lung. 



When the Pigeon is standing, the alternate elevation and de- 

 pression of the sternum, produced partly by the abdominal, partly 

 by the intercostal muscles, causes an alternate enlargement and 

 diminution of the capacity of the coelome, and thus pumps air in 

 and out of the lungs. During flight, when the weight is supported 

 by the wings, and the sternum is thus rendered relatively immov- 

 able, the same effect seems to be produced by the elevation and 

 depression of the back. In either case the inspired air rushes 

 through the lungs into the air-sacs and thence by diffusion into 

 the pneumatic cavities of the bones. Thus, while in other animals 

 a certain amount of unchanged or residual air is always left in 

 the lungs after each expiration, in Birds the residual air is confined 

 to the air-sacs and to the smaller branches of the bronchi, every 

 respiratory movement drawing a current of fresh or tidal air 

 through the lungs. As a result of this the aeration of the blood is 

 very complete and its temperature correspondingly high. It is 

 worthy of notice that Birds agree with Insects, the only other 

 typically aerial class, in having the inspired air distributed all 

 over the body, so that the aeration of the blood is not confined 

 to the limited area of an ordinary respiratory organ. 



Circulatory Organs. The heart (Fig. 1051) is of great pro- 

 portional size, and, like that of the Crocodile, consists of four 

 chambers right and left auricles, and right and left ventricles. 

 There is no sinus venosus, that chamber being, as it were, absorbed 

 into the right auricle (Fig. 1054, A, r. au.). The right ventricle 

 (Fig. 1054, B) partly encircles the left, the former having a crescentic, 

 the latter a circular cavity in transverse sections. The left 

 auriculo-ventricular valve has the usual membranous structure, 

 consisting of two flaps connected with the wall of the ventricle by 

 tendons, but the corresponding valve of the right side (V.) is a 

 large muscular fold, very characteristic of the class. 



The right auricle receives the right and left precavals (r. prc., 

 pc. v.) and the postcaval (ptc.) ; the left four large pulmonary veins 

 (p. v.). The left ventricle (Fig. 1055, I vn.), as in the Crocodile, 

 gives origin to the right aortic arch (a. ao.), but the right ventricle 

 (r. vn.) gives off only one trunk, the pulmonary artery, which soon 

 divides into two (r.p.a., l.p.a.). The left aortic arch is absent in 

 the adult, and it is the right alone which is continued into the 

 dorsal aorta. The result of this is that the systemic arteries receive 

 pure arterial blood from the left side of the heart, and the only 

 mingling of aerated and non-aerated blood is in the capillaries. 

 This is perhaps the most important physiological advance made 

 by Birds over Reptiles. 



