COLUMBA 127 



lead to the lungs which are closely pressed up against the ribs. 

 The cavity of the lungs is repeatedly subdivided, giving them 

 the appearance of being filled with a very vascular and spongy 

 material. They are no longer simple hollow sacs with large 

 undivided cavities as in the lower forms. The lungs of the 

 bird are peculiar in that they give off a number of pouches or 

 air-sacs, which extend into many parts of the body. There 

 are nine of these air-sacs, arranged in the following manner : 

 a pair of cervical sacs at the base of the neck on each side ; an 

 interclavicular sac in the region of the furcula ; two pairs of 

 thoracic sacs, and a pair of abdominal sacs. The bronchi lead 

 through the lungs, into which they give off a few air-passages, 

 to the air-sacs. The walls of the air-sacs are not vascular and 

 no respiratory exchange takes place in them ; they act as 

 reservoirs, and when the body cavity is compressed by raising 

 the sternum, the air in them is forced into the lungs and out 

 again. The efficiency of the respiratory system of the bird is 

 due to the fact that there are no blind ends, and the air in the 

 spaces of the lungs is completely refreshed at each expiration. 

 It is worth noticing that the temperature of the body of birds 

 is remarkably high (about 42 C), and this is connected with 

 the efficiency of the respiratory exchange. Expiration is the 

 active process, by dropping the sternum the air-sacs expand 

 and fill again. It may be remembered that several of the bones 

 are hollow, and diverticula of the air-sacs extend into them, 

 as for example those of the interclavicular sac into the humerus. 

 Vascular System. — The heart of the bird is not unlike that 

 of the lizard, but the ventricle is completely divided into two 

 by an interventricular septum. The pulmonary arch is 

 present in the bird just as it is in the lizard, and it leads from the 

 right ventricle to the lungs. The right aortic (or systemic) 

 arch is also present, arising from the left ventricle. The left 

 arch which in the reptile arises from the right side of the 

 ventricle and receives mostly venous blood, has disappeared 

 completely in the bird. All the venous blood returns to the 

 heart from the superior and inferior venae cava? direct into the 

 right auricle, there being no sinus venosus. All this blood 

 passes into the right ventricle and to the lungs, from which the 



