GENERAL CHARACTERS OF THE VERTEBRATA. 357 



veins to the right auricle. From the right auricle (fig. 13 1, «) the 

 blood passes by a valvular aperture into the right ventricle (v), 

 whence it is driven through the pulmonary artery to the lungs. 

 The right side of the heart is therefore wholly respiratory in its 

 function. Having been submitted to 

 the action of the lungs, and having 

 given off carbonic acid and taken up 

 oxygen, the blood now becomes arte- 

 rial, and is returned by the pulmonary 

 veins to the left auricle (a'). From 

 the left auricle the aerated blood passes 

 through a valvular aperture into the 

 left ventricle (»'), whence it is pro- 

 pelled to all parts of the body by 

 means of a great systemic vessel, the 

 " aorta." The left side of the heart is 

 therefore wholly occupied in, carrying 

 out the " greater" or systemic circula- 

 tion. 



The purification of the blood is car- 

 ried out in all Vertebrates by means 

 of distinct respiratory organs, assisted 

 to a greater or less extent by the skin. 

 In the Fishes, and in the Amphibians 

 to some extent, the process of respira- 

 tion is carried on by means of iran- 

 chice or gills — that is, by organs adapted 

 for breathing air dissolved in water. 

 These arCj therefore, often spoken of 

 as "Branchiate" Vertebrates; but the 

 Amphibians always develop true lungs 

 in the later stages of their existence. 

 In the Reptiles, Birds, and Mammals, 

 branchise are never developed, and the 

 respiration is always carried on by 

 means of true lungs — that is, by organs adapted for breathing 

 air directly. These are therefore often spoken of as the " Ab- 

 ranchiate" Vertebrates. 



The waste substances of the body — of which the most im- 

 portant are water, carbonic acid, and urea — are got rid of by 

 the skin, lungs, and kidneys. Under ordinary circumstances, 

 the lungs are mainly occupied with the excretion of carbonic 

 acid and watery vapour. The skin chiefly gets rid of super- 

 fluous moisture, but can also in many animals excrete qarbonic 



Diagram of the cir- 

 culation of a Mammal. The 

 venous system is marked 

 black ; the arterial system is 

 left white, a Right auricle ; 

 V Right ventricle ; p Pulmo- 

 nary artery, carrying venous 

 blood to the lungs; pv Pul- 

 monary veins carrying arterial 

 blood from the lungs ; a! Left 

 auricle ; 7/ Left ventricle ; b 

 Aorta, carrying arterial blood 

 to the body; c Vena cava 

 carrying venous blood to the 

 heart. 



