276 HUMAN PHYSIOLOGY 



also pressed upon, and the sustained contraction of the limb 

 muscles tends to prevent the blood flowing through them. 

 It is thus forced to the central nervous system in which the 

 pressure rises, and, if a weak spot in the vessels is -present, 

 rupture is apt to occur. But, if the effort is still further 

 sustained, the high intra-thoracic pressure tends to prevent 

 proper diastolic filling of the heart ; blood is therefore not 

 sent on from the veins into the arteries, the veins become 

 congested and the arterial pressure falls, less blood goes to 

 the brain, and thus fainting may result. 



In man the position of the abdominal reservoir of blood 

 at a lower level than the heart increases the work of that 

 , organ. For this reason, in people with a weak heart, the 

 sudden assumption of the erect position may lead to a failure 

 of the heart and to fainting. Especially is this the case when 

 the abdominal wall is lax, so that accumulation of blood in 

 the abdominal vessels is not prevented. In the recumbent 

 position, when the reservoir is on the same level as the pump, 

 the work is much easier. 



On the other hand, in the " head down position," the accu- 

 mulation of blood in the dependent parts is prevented in 

 the head by the vessels being packed inside the skull and in 

 the right side of the heart by the supporting pericardium. 



The Time taken by the Circulation. 



This was first determined by injecting ferrocyanide of 

 potassium into the proximal end of a cut vein, and finding 

 how long it took to appear in the blood flowing from the 

 distal end. From observation in the horse, dog, and rabbit, 

 it appears that the time corresponds to about twenty-seven 

 beats of the heart, so that in man it should amount to about 

 twenty-three seconds. 



Stewart has investigated the rate of flow through different 

 organs by injecting salt solution into the artery, and by 

 detecting its appearance in the vein by the change in the 

 electric conductivity of the contents of the vessel. 



