MAMMALIA. oF 
The organs of circulation may be double, so that all the blood conveyed 
Sy the veins from the different parts, must undergo a process of circulation, 
before 1t can be returned by the arteries; or they may be simple, in which 
case, only a portion of the blood which returns to the body, passes through 
the lungs. This last is the case with reptiles. The quantity of their 
respiration, and the qualities depending on it, vary with the relative propor- 
tion of blvod, returned at each pulsation, into the lungs. 
Fishes have a double circulation; but as they respire through the medium 
of water, and their blood only receives the portion of oxygen in that 
medium, their quantity of respiration is, perhaps, less than that of reptiles. 
In the Mammalia, the circulation is double, and the respiratory process 
simple. The quantity of their respiration is superior to that of reptiles and 
fishes. But the quantity of respiration in birds is still greater than that 
of quadrupeds, because they also respire by various other cavities, as well 
as the lungs. The air penetrates through their whole body, and acts upon 
the branches of the aorta, with the same efficiency as upon those of the 
pulmonary artery. 
From these circumstances result four different kinds of motion, among 
vertebrated animals. Quadrupeds, in whom the quantity of respiration is 
moderate, are formed for walking and running, and their predominant 
characteristic is vigor. Birds, whose respiratory system is more extensive, 
possess the lightness and strength of muscles necessary to support them in 
their flight. Reptiles, which respire more feebly, creep upon the earth, and 
many of them pass more or less of their existence in a state of torpor. And 
fishes, which move ina fluid almost as specifically heavy as themselves, 
are enabled io execute their movements, by an arrangement altebether 
different from the others. Every peculiarity of organization proper to 
each of these classes, and especially such as belong to motion and external 
sensation, have a close and necessary relation with the characters now 
enumerated. 
CLASS FIRST—MAMMALIA. 
Vertebrated Animals, with red and warm blood, breathing through lungs, vivi- 
parous, and suckling their young with milk formed in their breasts, or 
mamme. 
Tue class Mammalia is placed at the head of the Animal kingdom, not 
only because it is the class to which Man, considered in his animal structure, 
belongs, but also bec use the Mammalia enjoy the most numerous faculties, 
the most delicate sensations, and the most varied powers of motion. As the 
quantity of respiration is in mammiferous animals moderate, taey are 
generally formed for walking, and, in consequence, all the articulations of 
the:r frame have defined forms, which determine their motions. 
