INTRODUCTORY. 
17 
veyed by the arteries to the different parts of the body. In the 
three first classes of animals with vertebra;, the lungs consist of 
a number of small cells, through which the external air passes 
on inhalation. 
The quantity of respiration depends on two causes: first, 
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 effective 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 mammae, or breasts. It is 
from the teats that the class takes its name. 
The Mammalia, having but a moderate respiration, are 
a 3 
