Chap. III.] GENERAL ANATOMY. 49 



The lungs are flattened and oval in shape, and are firmly fixed 

 on each side of the vertebral column. Each is made up of a 

 close plexus of air-tubes connected with the branches of the 

 bronchi. There is a direct communication through the lungs 

 with the air-sacs. The muscular fibres of the so-called dia- 

 phragm, which is totally different from that of the rabbit, arise 

 from the ribs, outside the margins of the lungs, and from the 

 vertebral column, and are attached to a strong aponeurosis 

 (flattened shining masses of tendinous connective tissue) upon 

 the ventral surface of the lungs. 



The mechanism of respiration differs from that of either the 

 frog or the rabbit. In the frog the lungs are inflated by a 

 buccal force-pump ; in the rabbit they are inflated by a thoracic 

 suction-pump ; in the pigeon they cannot be inflated at all. It is 

 the air-sacs which are inflated, the air being sucked into them 

 through the lungs, as the thoracic cavity is enlarged by move- 

 ments of the ribs and sternum. Thus air passes backwards and 

 forwards through the lungs with scarcely any inflation of the lungs 

 themselves. The structure and mechanism of the lung-system are 

 as perfect as that of the rabbit. But they are on a different plan. 



3. The Heart and Circulatory System. The heart, which lies in 

 its pericardial cavity, beneath the anterior end of the sternum, 

 is very large, and has four distinct chambers. The right ventricle 

 (Fig. 21, r. v.) is smaller and thinner- walled than the left (/. v.). 

 It gives off the pulmonary artery (pul. a.), which at once forks 

 to send off a right and left branch to the corresponding lung. 

 From the firmer larger left ventricle (1. v.) proceeds the aortic 

 arch (ao.), and to the left of this two large (innominate) arteries 

 (in. a.), which diverge like a V. Each sends off branches to the 

 head (carotid, c. c.), to the wing (brachial, br. a.), and to the 

 muscles of the breast (pectoral, pc. a.). The arch itself, which is 

 smaller than the innominate arteries, passes back over the right 

 bronchus, and represents the right arch of the frog, not the left 

 as in the rabbit. At the back of the abdominal cavity it 

 becomes the dorsal aorta, which gives off three branches to supply 

 the stomach and intestines, two in front of (cofliac, coe. a., and 

 anterior mesenferic, a. m. a.) and one behind the kidneys (posterior 

 4 



