THE HIGHER ORGANISMS 143 



place the blood is discharged in such manner that the 

 freshly aerated portion rushes into the aorta and to the 

 head and brain of the animal, while the exhausted blood 

 later follows the greater part of it going to the lungs. 



Similar primitive arrangements obtain among the 

 batrachia, generally and also among the reptilia with 

 the exception of the crocodiles. The reptilian heart is 

 somewhat improved over that of the batrachia, but 

 it is only in the crocodiles that the ventricle becomes 

 completely divided by a septum. 



In birds and mammals the circulatory system attains 

 perfection in the sense that the heart is completely four- 

 chambered and is thus able to effect a complete separa- 

 tion of the aerated or arterial from the exhausted or 

 venous blood. In these animals the blood leaving the 

 left ventricle is distributed by the aorta and its branches 

 to the entire systemic circulation where it passes through 

 the capillaries and is gathered together in two large 

 veins, or vence cavce, which convey it into the right auricle. 

 From the right auricle it passes into the right ventricle, 

 and thence through the pulmonary artery to the lungs. 

 Having passed through the pulmonary capillary plexuses 

 for aeration, it is collected in several pulmonary veins 

 by which it is returned to the left auricle, from which it 

 passes into the left ventricle and again makes the sys- 

 temic circuit. By these means a certain quantity of 

 freshly aerated blood is constantly being distributed to 

 the viscera for the support of their cells, while an equal 

 quantity is always being sent to the lungs to be freshly 

 aerated. The two circulations being independent of 

 one another, no opportunity is ever afforded for the 

 venous and arterial bloods to mix. 



The Respiratory System. Respiration being an indis- 

 pensable function of living substance early requires 

 special means by which it shall be made possible for 

 all the cells of the composite animal to receive oxygen. 



The unicellular organisms whose activities and require- 

 ments typify those of the cells of the higher composite 



