38 INTRODUCTION. 



Respiration is partial, and circulation simple in reptiles, in 

 which there is only one ventricle and one aorta, of which the 

 pulmonary artery is a branch. In all other animals which 

 have a local respiration and a circulation, this latter is double, 

 and respiration complete; i. e. at every circuit of the blood, 

 the whole liquid passes through the respiratory organs. In 

 the articulated animals and mollusca, the circle is simple; in 

 the former the blood goes from the heart to the whole body, 

 and passes entirely through the branchiae; the same is the 

 case in fishes; in the mollusca, it goes from the heart to the 

 branchiae, passing first through the whole body. In birds and 

 the mammalia, the two hearts being joined, the circle is dou- 

 ble, or rather, the circuit is crossed, and ma} r be represented 

 by the figure 8, at the centre of which is the heart. 



22. The nutritive fluid must not only be submitted to 

 the action of the atmosphere, but must also be freed by the 

 secretions, from superfluous matters. In animals which have 

 an internal cavity, and consequently two surfaces, these two 

 surfaces, in all their extent, serve for the purpose of excretion 

 as well as of absorption. The internal and external skin pre- 

 sent also small cavities or particular depressions from which 

 the liquid issues. Finally, even in the animals in which there 

 is no circulation, if some particular liquid is to be produced, 

 the cavities or depressions either internal or external, of the 

 skin, are prolonged and ramified into the body in the form of 

 vessels or excretory canals, and take up from the nutritive 

 fluid, the elements proper for the composition of this liquid. 

 In the same manner, in the animals which have a circulation, 

 the vessels sometimes spread simply over large surfaces, and 

 permit the secreted fluid to escape by perspiration; at others it 

 is from the bottom of small cavities or follicles formed either in 

 the internal or external skin that the liquid oozes; in other parts, 

 the arteries, at the point where the arteries change into veins, 

 communicate with ramified excretory canals which are always 

 formed by the internal or external skin; and from the union 

 and combination of these canals with the blood vessels, result 

 the glands. These last organs of secretion are peculiar to those 

 animals which have a heart The liver, for instance, which is 



