90 CIKCULATION OF THE BLOOD. 



rapid, until, at the expiration of twenty minutes, it had entirely ceased. The entire 

 coating of collodion was then instantly peeled off. Quite a rapid circulation immediately 

 commenced, but it soon began to decline and in twenty minutes had almost ceased. In 

 another observation, the coating of collodion was applied without destroying the medulla. 

 The circulation was affected in the same manner as before and ceased in twenty-five 

 minutes. These experiments, taken in connection with observations on the influence of 

 asphyxia upon the arterial pressure, conclusively show that non-aerated blood cannot 

 circulate freely in the systemic capillaries. Venous blood, however, can be forced 

 through them with a syringe, and, even in asphyxia, it slowly filters through. 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 ventricle are evi- 

 dently capable of giving an impulse to the blood in the smallest arterioles ; for a marked 

 acceleration 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 left ventricle. 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. Dr. Sharpey adapted a syringe, with a heema- 

 dynamometer 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 of mercury. It spurted out at the vein in 

 a full jet under a pressure of five inches. 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. This is much less than the arte- 

 rial 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 unnecessary to discuss the views of Bichat and some 

 others, who supposed that the action of the heart had no effect upon the capillary circu- 

 lation. It must be admitted that this is its prime cause ; 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 circulation. A distinct impulse, follow- 

 ing each ventricular systole, is observed in the smallest arteries ; the blood flows from 

 them directly and freely into the capillaries ; and there is not the slightest ground for 

 the supposition that the force is not propagated to this system of vessels. 



Various writers have supposed the existence of a " capillary power," which they have 

 regarded as of greater or less importance in producing the capillary circulation. The 

 ideas of some are purely theoretical, but others base their opinion on microscopical 

 observations. These views do not demand extended discussion. There is a force in 

 operation, the action of the heart, which is capable of producing the capillary circula- 

 tion ; and there is nothing in the phenomena of the circulation in these vessels, which is 

 inconsistent with its full operation. Under these circumstances, it is unphilosophical to 

 invoke the aid of the currents produced in capillary tubes in which liquids of different 

 characters are brought in contact, or a " capillary power " dependent upon a so-called 

 vital nutritive attraction between the tissues and the blood, unless we do it on the basis 

 of phenomena observed in the capillaries when the action of the heart is suppressed. 

 When the heart ceases its action, movements in the capillaries are sometimes due to the 



