EAPIDITY OF THE CAPILLARY CIRCULATION. 89 



is subject to such great variations, and the differences in different situations are so con- 

 siderable, 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 imprac- 

 ticable to estimate the capacity of the capillary as compared with the arterial system. 

 The rapidity of the flow of blood is by no means so great as it appears in microscopical 

 examinations, being, of course, exaggerated in proportion to the magnifying power 

 employed. It is, nevertheless, to microscopical investigations that we are indebted for 

 the scanty information we possess on this subject. The estimates which have been made 

 by various observers refer generally to cold-blooded animals and have been arrived at by 

 simply calculating the time occupied by a blood-corpuscle in passing over a certain dis- 

 tance. Hales, who was the first to investigate this question, estimated that, in the frog, 

 a corpuscle moved at the rate of an inch in ninety seconds. The estimates of Weber and 

 Valentin are considerably higher, being about one-fiftieth of an inch per second. Volk- 

 mann calculated the rapidity in the mesentery of the dog, which would approximate 

 more nearly to the human subject, and found it to be about one-thirtieth of an inch 

 per second. Yierordt 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. He states that when the eye is fatigued, and sometimes when the nervous 

 system is disordered, compression of the globe in a certain way will enable one to see 

 a current like that in a capillary plexus. This he believed to be the capillary circula- 

 tion, and, by certain calculations, he formed an estimate of its rapidity, putting it at from 

 one-fortieth to one-twenty-eighth of an inch per second. The latter figure accords pretty 

 nearly with the observations of Volkmann upon the dog. How far these observations 

 are to be relied upon, it is impossible to say. Certainly no great importance would be 

 attached to them if they did not, in their results, approximate to the estimates of Volk- 

 mann, which probably represent, more nearly than any, the rapidity of the current in 

 the capillaries of the human subject. After what has been said of the variations in the 

 capillary circulation, it is evident that the foregoing estimates are by no means to be 

 considered exact. 



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 cannot 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 respiration. The fact is evident that 

 arrest of respiration produces arrest of circulation. This is ordinarily attributed to an 

 impediment to the passage of blood through the lungs when they no longer contain the 

 proper quantity of oxygen. This view is entirely theoretical and has been disproved by 

 experiments dating more than half a century ago. In 1789, Goodwyn advanced the 

 theory that, in asphyxia, the blood passes through the lungs but is incapable of exciting 

 contractions in the left ventricle. Bichat, in his celebrated essay " Sur la vie et la mort" 

 1805, proved by experiment that black blood passes through the lungs in asphyxia and is 

 found in the arteries. His theory was that non-aerated blood, circulating in the capilla- 

 ries of the nervous centres, arrests their function, thus acting indirectly upon the circu- 

 lation ; and that finally the heart itself is paralyzed by the circulation of black blood in 

 its substance. 



The immediate effects of asphyxia upon the circulation are referable to the general 

 capillary system. This fact we demonstrated conclusively by experiments upon the frog, 

 published in 1857. In these experiments, the medulla oblongata was broken up, and the 

 web of the foot was submitted to microscopical examination. This operation does not in- 

 terfere with the circulation, which may be observed for hours without difficulty. The cu- 

 taneous surface was then coated with collodion, care only being taken to avoid the web 

 under observation. The effect on the circulation was immediate. It instantlv became less 



