THE VASOMOTOR NERVES. 613 



local chemical stimuli. Carbon dioxid causes a dilatation of the 

 capillaries presumably by virtue of its acidity. And certain sub- 

 stances like histamine have a marked paralyzing effect upon these 

 vessels, causing a long-lasting or permanent dilatation. A sugges- 

 tion has been made that through the combined action of a central 

 influence exerted through the nerves upon the capillaries and of 

 local chemical stimuh the capacity of the capillaries may be ad- 

 justed to the needs of the tissues. That is to say, through a 

 nervous influence the capillaries may be kept in a condition of 

 tone which may be lowered during functional activity of the tis- 

 sue, the muscles, for example, by the local effect of CO2 or other 

 products of metabohsm. The conception that the blood-supply 

 to the organs may be controlled by a nervous and chemical regula- 

 tion of the capillaries and veins as well as the arteries constitutes 

 a new departure in the physiology of the circulation and may be 

 very helpful in explaining certain vascular phenomena which 

 hitherto it has been difficult to understand.* In the condition 

 known as surgical shock, for example, the significant symptom is 

 a serious and perhaps irrecoverable fall in arterial pressure. It 

 was thought formerly that this fall is due to a paralysis of the vaso- 

 motor (arteriomotor) fibers, but investigation has shown that in 

 shock these fibers are still in good condition. More recently ex- 

 perimental evidence has been obtained which indicates that the 

 trouble is in the capillary area, these vessels are paralyzed and 

 dilated possibly as a result of the action of toxic substances formed 

 in the damaged tissues. When any large area of the capillary bed 

 is overdilated permanently one can understand that the result 

 will be a pooling or stagnation of the blood in that area, and that 

 in consequence of the diversion of this quantity of blood from the 

 general circulation the arterial pressure may fall below the level 

 essential for the nourishment of the tissues. 



* For a review of this subject with the literature see Hooker, "Physiological 

 Reviews," 1, 112, 1921. 



