MAMMALIA. 27 



The organs of circulation may be double, so that all the blood conveyed 

 by the veins from the different parts, must undergo a process of circulation, 

 before it 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 blood, returned at each pulsation, into the lungs. 



Fishes have a double circulation ; but as they respire through the medium 

 of Avater, 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 Avhole 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 in a fluid almost as specifically heavy as themselves, 

 are enabled to execute their movements, by an arrangement altogether 

 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 tvarm blood, breathing through lungs, vivi- 

 parous, and sncJding their young icith milk formed in their breasts, or 

 mamma. 



The 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 because 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, they are 

 generally formed for walking, and, in consequence, all the articulations of 

 their frame have defined forms, which determine their motions. 



