ORGANS OF RESPIRATION. 565 



cubation, and they continue open to the eighth. The aorta 

 divides from its origin, into two great lateral arches, each of 

 which subdivides into the five branchial arteries of its own 

 side, which receive the entire blood of the heart. Between 

 the second and third days of incubation, four pairs of 

 branchial arteries are observed to be produced in succession 

 from before backwards ; and the first, or anterior of these 

 pairs, has disappeared before the fifth pair of branchial 

 arteries is developed, behind the others, on the fifth day of 

 incubation. The same order of succession is observed in 

 the development of the pairs of branchial openings in the 

 neck, from the anterior to the posterior pair. The vibratile 

 cilia are seen in active motion, on the mucous lining of every 

 part of the respiratory organs of birds, from the openings of 

 the nostrils and the interior of the eustachian tubes, to the 

 minutest divisions of the bronchi, and the closed extremities 

 of the great air-sacs extending through the trunk. 



The respiratory organs of mammalia are confined to a 

 distinct thoracic cavity, separated from the abdomen by a 

 complete muscular and tendinous diaphragm ; and as this 

 function is not extended to their systemic vessels, as it is 

 in birds, they manifest a lower temperature of the body, and 

 a diminished energy of their muscular system, and of almost 

 all their vital properties. The lungs here are covered on all 

 sides with pleura, and float free in the cavity of the chest ; 

 they no longer communicate with abdominal air-sacs, or 

 with the cavities of the bones ; they are entirely divided 

 into more minute cells, so as to present a more extensive 

 respiratory surface than in inferior classes, and they are 

 larger in the male than in the female sex. They are more 

 minutely and equally divided into cells, and into lobes, and 

 present a larger respiratory surface, and occupy a larger 

 thoracic cavity, in the more powerful carnivorous quadrupeds, 

 than in the feebler herbivorous tribes, where the more 

 limited development of the respiratory organs is accompanied 

 with a diminished energy of all their movements and func- 

 tions. The contiguous minute terminal cells of the lungs 

 appear to communicate with each other, like the ultimate 

 tubuli of many other glands. The lung of the right side is 

 generally larger than the left, from the sinistral position of 

 the heart, especially in the higher mammalia and man^ and 



