INTaOflUCTORY 433 



the relative portion of blood contained at every instant of time, 

 in the respiratory organ ; and, secondly, the quantity of oxygen 

 which enters into combination with the surrounding fluid. 



In mammiferous animals the circulation is double, and is 

 performed by means of their capacious lungs alone. The vo- 

 lume of their respiration is consequently greater than in reptiles, 

 whose respiratory organs are less; and also greater than in 

 fishes, from their breathing through a more dense medium. 



Birds respire in greater volume than quadrupeds, in conse- 

 quence of their double circulation and aerial respiration, with 

 the addition of cavities, which penetrate through almost every 

 part of their bodies ; acting with the same elTective force upon 

 the branches of the aorta, as upon the pulmonary artery. 



Hence, in quadrupeds which are designed for walking and 

 running, the respiration is moderate. Birds which are lightly 

 formed, and which require strength of muscle to support them 

 in the air, have a greater degree of respiration. Reptiles, which 

 are destined to crawl along the earth, have a more restricted 

 respiration ; and fishes, which move through a fluid so much 

 specifically heavier than themselves, breathe by means of gills. 



OF THE MAMMALIA. 



Mammiferous animals are placed at the head of every sys- 

 tem, in consequence of the highest degree of organization being 

 allotted to them. The comparative perfection of their organs — 

 the number of their faculties — the delicacy of their sensations — 

 and their varied powers of motion, all combining to produce 

 a superior intelligence, which entitles them to a higher rank in 

 the scale of being. 



The young are produced alive, and nourished after birth by 

 milk, which is secreted within the mammie, or breasts. It is 

 from the teats that the class takes its name. 



The Mammalia, having but a moderate respiration, are 

 generally only fitted for progressive motion on the earth ; which 

 they can sustain with long endurance. A few species can, how- 

 ever, mount into the air, by means of extensible membranes, 

 attached to their limbs, which are usually much elongated ; as 

 in the bat tribe, &c. : while the cetaceous animals, and some 

 others, are formed to move in the water only, from the short- 

 ness of their limbs. 



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