68 CIRCULATION OF THE BLOOD 



the bulb was broken up and the web of the foot was submitted to micro- 

 scopical examination. The cutaneous surface was then coated with col- 

 lodion, care only being taken to avoid the web under observation. The 

 effect on the circulation was immediate. It instantly became less 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 began, but it soon declined and in twenty min- 

 utes had nearly ceased. In another observation, the coating of collodion 

 was applied without destroying the bulb. 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 on the arterial pressure, show that non-aerated blood can not 

 circulate freely in the systemic capillaries, probably on account of con- 

 traction of the muscular coat of the small arteries, due to irritation from 

 an excess of carbon dioxide in the blood. 



Causes of the Capillary Circulation. The pressure in the arteries, 

 which forces the blood toward the capillaries, is competent, unless 

 opposed by contraction of the arterioles, not only to send the blood 

 through the capillaries but to return it to the heart by the veins. 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 that is capable of producing the capillary circu- 

 lation ; and there is nothing in the phenomena of the circulation in these 

 vessels inconsistent with its full operation. 



Influence of Temperature on the Capillary Circulation. Within mod- 

 erate limits, a low temperature, produced by local applications, has been 

 found to dimmish the quantity of blood sent to the capillaries and retard 

 the circulation, while a high temperature increases the supply of blood 

 and accelerates its current. Poiseuille found that when a piece of ice 

 was applied to the web of a frog's foot, the mesentery of a small warm- 

 blooded animal or to any part in which the capillary circulation could 

 be observed, the number of corpuscles circulating in the arterioles 

 was much diminished, " those which carried two or three rows of cor- 

 puscles giving passage to but a single row." The circulation in the 

 capillaries first became slower and then entirely ceased in parts. On 

 removing the ice, in a very few minutes the circulation regained its 

 former characters. When, on the other hand, the part was covered 

 with water at 104 Fahr. (40 C.), the rapidity of the current in the 

 capillaries was so much increased that the form of the corpuscles could 

 with difficulty be distinguished. 



