2i6 Descriptive Zoology. 



the ceca. The terminal portion of the digestive tube is 

 widened, and is called the cloaca. It receives three sets of 

 products, (i) the residue of digestion, (2) the eggs, and 

 (3) the excretion of the kidneys. 



The Circulatory System of the Pigeon. The circula- 

 tion of blood is very rapid in birds because of their great 

 activity. The heart is proportionally large ; it is com- 

 pletely divided into two halves, so that the blood moves 

 through one half of the heart from the lungs to the body, 

 and through the other half from the body to the lungs, 

 being pumped twice in the circuit instead of once as in 

 fishes. An important point to note is that the aorta turns 

 to the right instead of to the left as in our bodies. 



How the Pigeon Breathes. Respiration is exceedingly 

 active in a bird. Imagine yourself taking such violent ex- 

 ercise as that required for a bird to fly through the air at 

 the rate of a mile a minute. Would you not be "out of 

 breath " ? The bird's lungs are of fair size ; in addition 

 there are air sacs in all parts of the body, communicating 

 with the lungs, the bronchial tubes continuing on through 

 the lungs into these sacs. The lungs do not lie free in the 

 body cavity as in our bodies, but are attached to the dorsal 

 wall, fitting closely between the ribs. The air sacs are also 

 in communication with the hollows of the bones, which 

 adds to the buoyancy. The temperature of birds is higher 

 than that of any other animals, being about 110 F. 



The movements of respiration in birds is peculiar, in that 

 expiration is accomplished by active muscular effort, and 

 inspiration by elastic reaction, just the opposite of the 

 processes of our bodies. 



The Excretory System of the Pigeon. The lungs serve 

 as organs of excretion, throwing off carbon dioxid. The 





