372 GENERAL ZOOLOGY 



presence of two caeca. The spleen is bright red, and is 

 attached to the walls of the glandular stomach. 



The Circulatory System. The heart is a large, four- 

 chambered organ inclosed in the pericardium (Fig. 195). 

 It consists of two auricles and two ventricles, the latter sep- 

 arated by a complete partition. The circulation is double, 

 and the aerated and nonaerated blood come nowhere in 

 contact, except in the capillaries. The blood is sent to the 

 lungs from the right ventricle, through the pulmonary artery. 

 Freed of its carbon dioxide, the blood returns through the 

 pulmonary vein to the left auricle, whence it passes to the 

 left ventricle and thence into the aorta, 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 Respiratory System. The organs of respiration are the 

 larynx, which opens out to the pharynx by a slit-like glottis ; 

 the trachea ; the bronchial tubes, which ramify through the 

 tissue of the lungs ; and the lungs themselves. 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, the organ of voice. The well-known 

 sounds are produced by the vibration 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 com- 

 municating 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 accomplished 

 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 complete, and a prac- 

 tically constant high temperature, 37° C. (100° F.), is main- 

 tained. The pigeon is therefore said to be warm-blooded. 



