CIRCULATION IN THE CAPILLARIES. 85 



and be weighted until the subjacent capillaries are emptied, an approximate 

 idea of the blood-pressure in the vessels may be obtained. Experiments 

 made in this way, by Von Kries, show that the pressure in the capillaries of 

 the hand raised above the head is equal to a little less than one inch (24 mm.) 

 of mercury ; in the hand hanging down, a little more than two inches (54 

 mm.) ; and in the ear, about 0*8 of an inch (20 mm). 



Rapidity of the Capillary Circulation. The circulation in the capillaries 

 of a part is subject to such great variations and the differences in different 

 situations are so considerable, that it is impossible to give any definite rate 

 which will represent the general rapidity of the capillary circulation. It is 

 for this reason that it has been found impracticable to estimate accurately 

 the capacity of the capillary as compared with the arterial system. In view 

 of the great uncertainty in the methods employed in the estimation of the 

 rapidity of the flow of blood in the capillaries, it seems unnecessary to discuss 

 this question fully. Volkmann calculated the rapidity in the mesentery of 

 the dog and found it to be about one-thirtieth of an inch (0-85 mm.) per 

 second. Vierordt made a number of curious observations upon himself, by 

 which he professed to be able to estimate the rapidity of the circulation in 

 the little vessels of the eye ; and by certain calculations, he formed an esti- 

 mate of its rapidity, putting it at one-fortieth to one-twenty-eighth of an 

 inch (0'63 and O9 mm.) per second, which estimate may be provisionally 

 adopted as the probable rate in the human subject. 



Relations of the Capillary Circulation to Respiration. In treating of the 

 influence of respiration upon the action of the heart, the arterial pressure, 

 pulse etc., it has already been stated that non-aerated blood can not circulate 

 freely in the capillaries. Various ideas with regard to the effects of asphyxia 

 upon the circulation have been advanced, which will be again discussed in 

 connection with the physiology of respiration. It is well known that arrest 

 of respiration produces arrest of circulation. 



The immediate effects of asphyxia upon the circulation are referable to 

 the general capillary system. In a series of experiments made in 1857, the 

 medulla oblongata was broken up, and the web of the foot was submitted to 

 microscopical examination. This operation does not interfere with the cir- 

 culation, which may be observed for hours without difficulty. The cuta- 

 neous surface was then coated with collodion, 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 began to de- 

 cline and in twenty minutes had almost ceased. In another observation, the 

 coating of collodion was applied without destroying the medulla. The cir- 

 culation was affected in the same manner as before and ceased in twenty- 

 five minutes (Flint). These experiments, taken in connection with observa- 

 tions on the influence of asphyxia upon the arterial pressure, show that non- 

 aerated blood can not circulate freely in the systemic capillaries. Non- 

 aerated blood, however, can be forced through them with a syringe, and 



