AN INTRODUCTION TO ZOOLOGY 



heart, taken thence to the lungs, back again to the left side of the 

 heart, and thence to the body. Here, again, there is no alternative 

 course ; to reach the same point again any given portion of blood 



must return twice to the 

 heart, it must pass through 

 both the pulmonary and 

 systemic circulations. Such 

 a condition we characterise 

 as a complete double circu- 

 lation. Lastly, in Rana, we 

 have a condition in some 

 respects intermediate be- 

 tween these two. While 

 we have both a pulmonary 

 and a systemic circulation, 

 each related to a separate 

 atrium, yet there is a possi- 

 bility of a mixture of blood 

 in either the ventricle or 

 the conus arteriosus. It is 

 conceivable, therefore, that 

 any given portion of blood 

 might only go to the heart 

 once before returning again 

 to the same point. We 

 indicate this by describing 

 the blood circulatory system 

 of the frog as incompletely 

 double. 



As in all land-dwelling 

 vertebrates the blood in a 

 mammal is kept circulatory 

 through the lungs and the 

 body by means of the 

 alternate contraction and 

 expansion, the two con- 

 ditions passed through 

 being termed systole and 

 diastole respectively. The 

 former commences practi- 

 cally simultaneously in the two atria, and drives the blood from 

 these chambers into the ventricles. When these latter are rilled 

 and the pressure in them exceeds that in the atria, the blood 

 attempts to flow back, but is prevented by the closure of the 



FIG. 109. Diagram illustrating the circu- 

 lation. From Furneaux. 



i, right atrium ; 2, left atrium ; 3, right ventricle ; 

 4, left ventricle ; 5, vena cava superior ; 6, vena cava 

 inferior ; 7, pulmonary arteries ; 8, lungs ; 9, pul- 

 monary veins; 10, aorta; u, alimentary canal ; 12, 

 liver ; 13, hepatic artery ; 14, portal vein ; 15, hepatic 

 vein. 



