94 THE CIRCULATION OF THE BLOOD 



The question arises, what has happened to the blood ? 

 It is not in the arteries, for these are constricted ; it is not 

 in the veins, for surgeons testify that these are not dilated. 

 It must therefore be in the capillaries. In the paralysed 

 and greatly distended capillaries a large proportion of the 

 blood is accommodated. The blood corpuscles are to a 

 great extent immobihsed, hke railway wagons on a siding. 

 Secondary changes then occur owing to the deficient oxida- 

 tion of the tissues. The stagnated blood, too, becomes 

 concentrated in corpuscles owing to the excessive transuda- 

 tion of plasma into the tissue-spaces. 



As regards the cause of the capillary paralysis, it has been 

 found, as the result of observations on men wounded 

 in the late War, that a relationship exists between the 

 tendency to shock and the degree to which muscle is 

 involved in the injury. Shock can indeed be produced 

 experimentally by crushing muscles. It is therefore beheved 

 that substances resembling histamine in action are, in the 

 destruction of tissue, hberated into the blood-stream. 

 These paralyse the capillaries and lead to the stagnation 

 of blood above described. 



To what degree the capillaries, hke the arterioles, are 

 under nervous control is not determined. It is possible 

 that the antidromic impulses which we have seen to con- 

 stitute axon reflexes travel to the capillaries, and not 

 merely to the arterioles. 



THE CIRCULATION IN THE VEINS 



In the veins the blood-pressure is 10 mm. Hg., and lower 

 as the heart is approached. The blood is driven along the 

 veins by two forces : the pressure of the blood behind it — 

 that is to say, the kinetic energy communicated to it by the 

 contraction of the left ventricle — and the negative pressure 

 in front created by the contraction and relaxation of the 

 right auricle. Two accessory factors combine in giving a 

 further impetus to the venous flow. The first consists of 



