GENERAL CHARACTERS OF THE VERTEBRATA. 335 



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 sub- 

 mitted to the action of the lungs, 

 and having given off carbonic acid 

 and taken up oxygen, the blood 

 now becomes arterial, and is re- 

 turned by the pulmonary veins to 

 the left auricle (a 1 ). From the left 

 auricle the aerated blood passes 

 through a valvular aperture into the 

 left ventricle (if), 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 occu- 

 pied in carrying out the " greater" 

 or systemic circulation. 



The purification of the blood 

 is carried 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 

 respiration is carried on by means 

 of branchice 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 " Abranchiate " 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 superflu- 

 ous moisture, but can also in many animals excrete carbonic 

 acid as well. The kidneys are present in almost all Vertebrate 



Fig. 123. Diagram of the circulation 

 of a Mammal. The venous system 

 is marked black ; the arterial system 

 is left white. a Right auricle; v 

 Right ventricle ; p Pulmonary artery, 



gs; 



u' Left ventricle ; & Aorta, carrying 

 arterial blood to the body ; c Vena 

 cava carrying venous blood to the 

 heart. 



