86 CIRCULATION OF THE BLOOD IN THE VESSELS. 



even in asphyxia, it passes slowly into the veins. If air be admitted to the 

 lungs before the heart has lost its contractility, the circulation is restored. 

 No differences in the capillary circulation have been noticed accompanying 

 the ordinary acts of inspiration and expiration. 



Causes of the Capillary Circulation. The contractions of the left ventri- 

 cle are evidently capable of giving an impulse to the blood in the smallest 

 arterioles ; for a marked acceleration of the current accompanying each sys- 

 tole 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 left ventricle. This, however, can not rigidly be applied to the nat- 

 ural circulation, as the smallest arteries during life are endowed with con- 

 tractility, which is capable of modifying the blood-current. Sharpey adapted 

 a syringe, with a ha3madynamometer attached, 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 (89 mm.) of mercury. It spurted out at the vein in a full 

 jet under a pressure of five inches (127 mm.). In this observation, 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. 



It is thus seen that the pressure in the arteries which forces the blood 

 toward the capillaries is competent, unless opposed by contraction of the 

 arterioles, not only to cause the blood to circulate in the capillaries, but to 

 return it to the heart by the veins ; and the only questions to be considered 

 are first, whether there be any reason why the force of the heart should not 

 operate on the blood in the capillaries, and second, whether there be any 

 force in these vessels which is superadded to the action of the heart. The 

 first of these questions is answered by microscopical observations on the cir- 

 culation. A distinct impulse, following each ventricular systole, is observed 

 in the smallest arteries ; the blood flows from them directly and freely into 

 the capillaries ; and there is no ground for the supposition that the force is 

 not propagated to this system of vessels. There is, therefore, a force, the 

 action of the heart, which is capable of producing the capillary circulation ; 

 and there is nothing in the phenomena of the circulation in these vessels 

 which is inconsistent with its full operation. When the heart ceases its 

 action, movements in the capillaries are sometimes due to the contractions of 

 the arteries, an action which has already been fully described. Movements 

 which have been observed in membranes detached from the body were un- 

 doubtedly due to the mere emptying of the divided vessels or to simple gravi- 

 tation. 



There is a circulation of the blood in the area vasculosa, the first blood- 

 vessels that are developed before the heart is formed ; but there are no defi- 

 nite and reliable observations which show that there is any regular movement 

 of the blood, which can be likened to the circulation as it is observed after 

 the development of the heart, anterior to the appearance of a contractile cen- 

 tral organ. Another example of what is supposed to be circulation without 



