292 THE CIRCULATION OF THE BLOOD 



dent return of venous Hood to the heart. In the first place, let us see 

 whether shock can be produced experimentally in animals by mechanical 

 interference with the bloodflow in the vena cava. That such is the case 

 was shown by H. H. Janeway and Jackson, 31 who found that mechanical ob- 

 struction of the inferior vena cava for a short time was followed by the usual 

 signs of shock. Such interference with the venous return to the heart 

 may also be produced by excessive movements of the thorax as a re- 

 sult of artificial respiration. That this in itself may cause shock is known 

 to all experimental investigators on the subject, although the interpre- 

 tation has not always been that which is given above. Yandell Hen- 

 derson 32 thought that the excessive ventilation caused a blowing off 

 of carbon dioxide from the blood (see page 293), thus producing a 

 low tension of this gas in the blood (acapnia), which he believed to be 

 the responsible factor. 



As in gravity shock, so in surgical shock, stagnation of blood in the 

 splanchnic area is common; the animal bleeds into his own (splanchnic) 

 blood vessels (capillaries and venules), because these have lost their tone. 

 As we have noted above, one of the most certain ways of producing 

 shock is by exposure and rough handling of the abdominal viscera. It 

 is therefore of importance to study the effects that can be noted on 

 the blood vessels of this area under such conditions. When the viscera 

 are first exposed to air, there may be a short period during which vaso- 

 constriction is evident. This is soon followed by a dilatation of the 

 arterioles in the exposed area, causing the capillaries and veins to be- 

 come markedly distended as during the first stage of inflammation. This 

 accumulation of blood in the mesenteric veins has been shown by Mor- 

 rison and Hooker to cause an increase in the weight of an isolated loop 

 of intestine as an animal passes into a state of shock. 



Splanchnic engorgement alone does not, however, suffice to explain all 

 the loss of blood, and we are driven to conclude that the capillaries of 

 the tissues outside the abdomen must entrap much of it. As a matter of 

 fact, Cannon, and others, have found that concentration of the blood 

 occurs in these capillaries as indicated by comparisons of the corpuscles 

 and hemoglobin in blood drawn from veins and from capillaries. Nor- 

 mally the values are equal; in shock the capillary blood is much con- 

 centrated. 



In so far as the circulatory disturbances are concerned, we may there- 

 fore sum up the conditions occurring in shock as follows: The blood 

 accumulates in the veins and capillaries that is, in a part of the vas- 

 cular system that is beyond vasomotor control. The consequent with- 

 drawal of this blood from the circulation produces a diminution of the 

 bloodflow in the vena cava and consequently an inadequate filling of the 



