CAUSES OF THE CAPILLARY CIRCULATION. 293 



ticed accompanying the ordinary acts of inspiration and 

 expiration. 



Causes of the Capillary Circulation. The contractions 

 of the left ventricle are evidently capable of giving an im- 

 pulse to the blood in the smallest arterioles, for a marked ac- 

 celeration of the current accompanying each systole can be 

 distinguished in all but the true capillaries. It has also been 

 shown by experiments after death, that blood can be forced 

 through the capillary system and returned by the veins by a 

 force less than that exerted by the heart. This, however, 

 cannot rigidly be applied to the natural circulation, as the 

 smallest arteries are endowed during life with contractility, 

 which is capable of modifying the blood current. 1 Dr. 

 Sharpey adapted a syringe, with, a hemodynamometer at- 

 tached, to the aorta of a dog just killed, and found that fresh 

 defibrinated blood could be made to pass through the double 

 capillary systems of the intestines and liver, by a pressure of 

 three and a half inches of mercury. It spurted out at the 

 vein in a full jet under a pressure of five inches. In this ob- 

 servation, the aorta was tied just above the renal arteries. 

 The same pressure, the ligature being removed, forced the 

 blood through the capillaries of the inferior extremities. 2 

 This is much less than the arterial pressure, which is equal 

 to from five and a -half to six inches of mercury. 



It is thus seen that the pressure in the arteries which 

 forces the blood toward the capillaries is competent, unless 

 opposed by excessive contraction of the arterioles, not only to 

 cause the blood to circulate in these vessels, but to return it to 

 the heart by the veins. This fact is so evident, that it is un- 



1 As showing the difference between the vessels immediately after death, and 

 after they have lost all their vital properties, we may refer to an observation of 

 Berard (op. cit., p. 776), in which he found it impossible to inject, with a soliditi- 

 able fluid, parts of the body immediately after amputation. Water passed with 

 facility, but alcohol or vinegar could not be forced through. 



2 TODD and BOWMAN, The Physiological Anatomy and Physiology of Man, 

 Philadelphia, 1857, p. 678. 



