CLASS AVES. BIRDS 



439 



leads into the cloaca, into which the urinary 

 and genital ducts also open. The cloaca 

 opens to the outside by means of the cloacal 

 opening or vent. In young birds a thick 

 glandular pouch of lymphatic tissue, the 

 bursa Fabricii, lies just above the cloaca. 



The large hvcr is bilobed and has two 

 bile ducts. There is no gall bladder in the 

 common pigeon, though it is present in some 

 birds and even in some species of pigeons. 

 The pancreas pours its secretions into the 

 duodenum through 3 ducts. A spleen and, in 

 young pigeons, paired thymus glands are 

 present. Lymph nodes are absent in birds 

 except in waterfowl. 



Circulatory system 



The heart of a bird (Fig. 310) is com- 

 paratively large. It is composed of two en- 

 tirely separated, muscular ventricles and two 

 thin-walled atria (p. 340). The right atrium 

 (Fig. 310) receives nonoxygenated venous 

 blood from the right anterior vena cava, the 

 left anterior vena cava, and the posterior 

 vena cava veins. This blood passes from the 

 right atrium into the right ventricle and is 

 then pumped through the pulmonary artery, 

 which divides into right and left pulmonary 

 arteries, leading to the right and left lungs 

 respectively. 



The blood, after being oxygenated in the 

 lungs, returns through 4 large pulmonary 

 veins to the left atrium. It passes from the 

 left atrium into the left ventricle and is then 

 pumped through the right aortic arch, which 

 gives off the innominate arteries and con- 

 tinues as the dorsal aorta (Fig. 310). 



Circulation in birds is extremely rapid; the 

 blood, which may reach the high tempera- 

 ture of 102° to 112° F. in some perching 

 birds, is forced through the body by a heart 

 that beats several hundred times per minute 

 when at rest, and up to a thousand or more 

 in the canary when under stress. 



Contrasting the circulatory system of the 

 pigeon with that of the turtle, it should 

 be noted that the venous blood and arterial 



blood do not mingle in the heart of the 

 pigeon. The renal portal system of the 



pigeon has almost completely disappeared, 

 the blood being taken from the posterior 

 part of the body directly to the heart, and 

 not through the renal capillaries as in all 

 lower vertebrates. The jugular veins of the 

 pigeon are united just under the head by 

 a cross vein; this special adaptation enables 

 the blood to pass back to the heart from the 

 head whenever the neck becomes momen- 

 tarily twisted and one of the jugular veins 

 is blocked. 



Respiratory system 



The two lungs in birds (Fig. 311) are 

 assisted by a remarkable system of air sacs. 

 During inspiration, relaxation of the thoracic 

 and abdominal muscles allows elastic ex- 

 pansion of the thorax and abdomen. Air 

 enters the mouth cavity through the nostrils, 

 as in reptiles; it then passes through the 

 glottis into the trachea or windpipe, which 

 divides, sending a branch (bronchus) to 

 each lung. Each bronchus as it enters into a 

 lung breaks up into smaller tubes. To the 

 smaller bronchial tubes are attached several 

 large thin-walled air sacs (Fig. 311) that 

 extend out between organs in the body to 

 spaces in the neck region, and into cavities 

 of the larger bones. 



At inspiration, air rushes through the 

 bronchial tubes into the air sacs; then, dur- 

 ing expiration, the muscles of the thorax and 

 abdomen contract, forcing the air from the 

 air sacs through the lungs and trachea, and 

 out through the nostrils. The bird has a 

 high respiratory rate; for the pigeon it is 29 

 times per minute at rest as contrasted with 

 14 to 20 times per minute in man. This 

 rapid flow of air through the respiratory sys- 

 tem explains why the lungs of birds can be 

 so small when they have the highest oxygen 

 requirements of all animals because of their 

 high metabolic rate. 



The air sacs function principally as an 

 accessory breathing organ; they may also 



