THE DOMESTIC PIGEON 369 



of its carbon dioxide, the blood returns through the pulmonary 

 vein (Fig. 191, 30) to the left auricle (Fig. 191, 23), whence it 

 passes to the left ventricle (Fig. 191, 24) and thence into the 

 aorta (Fig. 191, 25), which distributes it to all parts of the 

 body. The blood from the body returns to the right auricle, 

 whence it enters the right ventricle, completing its circuit. 

 Lymph circulates through the body of the pigeon in vessels 

 of the lymphatic system. 



The organs of respiration are the larynx, which opens out 

 co the pharynx by a slit-like glottis (Fig. 191, 31) ; the trachea 

 (Fig. 191, 32); the bronchial tubes, which ramify through the 

 tissue of the lungs, and the lungs themselves (Fig. 191, 34). 

 The trachea is kept open by rings of cartilage in its wall. 

 At the junction of the bronchial tubes and trachea is a slight 

 enlargement, forming the syrinx (Fig. 191, 33), the organ of 

 voice. The well-known sounds are produced by the vibra- 

 tion of a fold of membrane at this place. Many of the bones 

 are hollow, and there is a system of air-sacs scattered through 

 the body and communicating with the bronchial tubes. By 

 these means the air available for respiration is greatly increased 

 and the weight of the body is lessened. Breathing is accom- 

 plished by movements of the muscles of the thoracic region, 

 by which air is driven almost completely out of the lungs at 

 each expiration. The aeration of the blood is very complete, 

 and a high temperature, 37 C. (100 F.) is maintained. 



The kidneys (Fig. 191, 35) are dark, three-lobed organs 

 fitting closely into cavities beneath the back-bone. The ureters 

 (Fig. 191, 36) open into the cloaca (Fig. 191, 37). 



The Skeletal System. The skull (Fig. 192, l) is large, with 

 a comparatively large, rounded cranium (Fig. 192, 2). The 

 articulation of the cranium with the first vertebra is made 

 by a single condyle, as in the reptiles. The vertebrae of the 

 neck or cervical region (Fig. 192, 5) are free; those of the 

 thoracic region (Fig. 192, 6), pelvic region (Fig. 192, 7), and 



