THE CIRCULATION TIME. 179 



sufficient alone to force the blood onward and back to the heart, 

 and we must seek other accessory factors to explain the venous 

 return. 



The veins are equipped with cup-shaped valves which permit 

 the passage of blood only in one direction, i. e., towards the heart. 

 Every movement of a muscle therefore squeezes some of the blood 

 onward. This massaging influence of the muscles is very im- 

 portant. Its absence accounts for the fact that it is impossible 

 to stand still for a long period of time without the limbs becom- 

 ing very painful, especially in the case of varicose veins, where 

 the valves of the veins are no longer functional, so that there is 

 nothing to prevent the blood from returning to the more depend- 

 ent positions. Another source of energy to the returning blood 

 is the aspiratory effect of the thorax at each inspiration. This 

 action will be considered in the study of the respiratory mechan- 

 ism. 



Circulation Time. The actual time which is taken for the 

 blood to traverse the circulatory system has been variously esti- 

 mated. Obviously such figures can give only average results, 

 since the distance through which blood to the arm must flow is 

 less than that to the legs. In general, it may be said that the 

 blood makes a complete circulation in from 25 to 30 beats of the 

 heart. The circulation through the lungs requires about one- 

 fourth of this time. 



That the velocity of the blood flow through different vessels 

 varies, is apparent from actual observations made on severing 

 them and actually observing the rate of outflow. The following 

 figures expressing the blood supply per minute to each hundred 

 grammes of organ have been determined experimentally: 



Leg 5 c. c. Liver (venous) .... 59 c. c. 



Head 20 " Liver (arterial) ... 25 " 



Stomach 21 " Brain 136 " 



Intestines 31 " Kidney... 150 " 



Spleen 58 " Thyroid 560 " 



The Effect of the Circulation on the Blood. If the circula- 

 tion of the blood through the vessels of the lung or the web of a 



