THE CHICKEN. 99 



are only partially distributed there. They are prolonged 

 to the orifices of openings that pass through the lungs, . 

 and are continued beyond them to large sacs, called 

 aerial sacs, which extend into the abdominal and tho- 

 racic cavities, and even into the neck and along the 

 spinal column. These sacs communicate with the in- 

 terior of the bones, which are hollow, so that the bird is, 

 so to say, filled with air. 



The circulatory apparatus has nothing specially pecu- 

 liar ; the blood-corpuscles are, however, elliptical, and the 

 arch of the aorta, which turns from right to left in the 

 dog and in man, curves from left to right in the fowl. 

 In the courses of the blood-vessels are numerous vascu- 

 lar plexuses in which circulation is energetic ; in the hen 

 an abdominal plexus furnishes the heat necessary for the 

 hatching of the eggs during incubation. The exceed- 

 ingly active circulation and respiration raise the temper- 

 ature of birds to 108 ; that is considerably above the 

 temperature of mammals, which is from 98 to 100. 



The skeleton of the chicken, like that of most birds, is 

 peculiar in that there is no marrow in the bones, these 

 being hollow, as we have seen. Besides this there are 

 numerous modifications having for their object the pre- 

 sentation of large surfaces for the origin and insertion 

 of the muscles of flight. Thus, the sternum has on the 

 median line a prominent ridge like the keel of a ship, 

 and this is the breast-bone. This ridge furnishes a" very 

 great surface for the insertion of the muscles of the 

 wing. The ribs are completely ossified, instead of having 

 a cartilaginous extremity, as in the dog and in man, and 

 each has towards the posterior third a bony prolongation 

 directed towards the back and resting on the next rib. 

 These peculiarities have the effect of making the tho- 



