338 ZOOLOGY 



which nearly all cells possess. It has been estimated 

 that there are about five millions of these corpuscles in a 

 cubic millimeter of human blood, but the number differs 

 according to environmental conditions, e.g., altitude. 

 The less numerous corpuscles are colorless (so-called 

 white corpuscles) and are amoeboid, resembling minute 

 protozoa, and like them capable of independent life and 

 movement, given a suitable environment. The func- 

 tions of these two types of corpuscles are entirely differ- 

 ent ; the red carry oxygen through the body, while the 

 white serve as policemen, attacking and destroying 

 bacteria and dead tissues. 



Thecircuia- 9- The blood is contained in a closed system of ves- 

 tory system se j s k nO wn as arteries, capillaries, and veins, and is pro- 

 pelled through them by the beating of the heart. Just 

 as the stomach is an enlargement of the alimentary canal, 

 so the heart is primitively a mere swelling of a blood 

 vessel, provided with muscular walls. It is so far inde- 

 pendent of the main nervous system that its contrac- 

 tions will continue when it is isolated from the body. 

 In fishes we find that the blood coming from the various 

 parts of the body is collected in a sinus venosus, which 

 has contractile walls. Thence we pass to the auricle or 

 first part of the heart proper, then to the ventricle. In 

 sharks and some other forms there is in addition a bulb 

 (conus arteriosus] with muscular contractile walls at the 

 beginning of the great blood vessel (the aorta) leaving 

 the ventricle. In the course of evolution the sinus 

 venosus and conus arteriosus lost their distinct charac- 

 ter and function, while the heart became divided longi- 

 tudinally, so that there were two auricles and two ven- 

 tricles. Now the blood received from the great vein or 

 veins enters the right auricle and, passing into the right 

 ventricle, is pumped into the lungs, from which it re- 



