CLASS AVES 585 



(a.ao), which gives off the innominate arteries (in.a) and then 

 continues as the dorsal aorta {d.ao). 



Contrasting the circulatory system of the pigeon with that 

 of the turtle, it should be noted that the venous blood and arterial 

 blood are not allowed to mingle in the heart of the pigeon. The 

 renal portal system of the pigeon has almost completely dis- 

 appeared, 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 (Fig. 475, ju) of 

 the pigeon are united just under the head by a cross vein {ju') ; 

 this enables blood to pass back to the heart from the head when 

 the neck becomes momentarily twisted so that one of the jugular 

 veins is stopped up. 



The Respiratory System. — The two lungs in birds are as- 

 sisted by a remarkable system of air-sacs. During inspiration, 

 the relaxation of the thoracic and abdominal muscles allows 

 the elastic expansion 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. The bronchi 

 communicate with nine large thin- walled air-sacs, which lie' 

 principally along the sides and dorsal surface of the body-cavity. 

 During expiration, the muscles of the thorax and abdomen con- 

 tract, forcing the air from the air-sacs, through the lungs and 

 trachea, and out of the nostrils. At each inspiration practically 

 all of the air in the lungs is renewed. 



The air-sacs enable the bird to breathe easily when in flight, 

 since air is forced into them during the rapid progress through 

 the atmosphere and out of them by the compression of the 

 pectoral muscles, which lower the wings. In man, violent move- 

 ments interfere with the alternate inspiration and expiration of 

 air. 



The trachea is held open by partially ossified cartilaginous 

 rings. Where the trachea divides into the two bronchi, it en- 

 larges to form the vocal organ, or syrinx, a structure peculiar to 



