VERTEBRATA: GENERAL CHARACTERS. 



455 



of Vertebrates, but a brief sketch may be given here of the 

 circulation, in its most complete form, as in a Mammal. In 

 such a case, the venous or impure blood, which has circulated 

 through the body and has parted with its oxygen, is returned 

 by the great veins to the right auricle. From the right auricle 

 (fig. 246, a) 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 func- 

 tion. Having been submitted to the 

 action of the lungs, and having given 

 off carbonic acid and taken up oxygen, 

 the blood now becomes arterial, and 

 is returned by the pulmonary veins to 

 the left auricle ('), From the left aur- 

 icle the aerated blood passes through 

 a valvular aperture into the left ven- 

 tricle (v r ), whence it is propelled 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 circulation. 



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 branchuz 

 or gills that is, by organs adapted 

 for breathing air dissolved in water. 

 These are 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, branchiae 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 " Abran- 

 chiate " 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, 



Fig. 246. Di 



of the rir- 



ig. 240. L>iagram of tne cir- 

 culation of a Mammal. The 

 venous system is marked 

 black ; the arterial system is 

 left white, a Right auricle ; 

 r Right ventricle j / Pulmo- 

 nary artery, carrying venous 

 blood to the lungs ; pv Pulmo- 

 nary veins, carrying arterial 

 blood from the lungs ; of Left 

 auricle ; & Left ventricle ; b 

 Aorta, carrying arterial blood 

 to the body; c Vena cava, 

 carrying venous blood to the 

 heart. 



