350 THE CIRCULATION. 



to accuracy, during the microscopic examination of transparent and 

 vascular tissues. The results obtained in this way by different observers 

 (Valentin, Weber, and Volkrnann), show that the rate of movement of 

 the blood through the capillaries is rather less than one millimetre per 

 second ; or about 5 centimetres per minute. Since the rapidity of the 

 current must be in inverse ratio to the entire calibre of the vessels 

 through which it moves, it appears that the united calibre of all the 

 capillaries must be not less than 300 times greater than that of the 

 arteries. It does not follow from this, however, that the whole quantity 

 of blood contained in the capillaries at any one time is so much greater 

 than that in the arteries ; since, although the united calibre of the capil- 

 laries is large, their length is very small. The effect of the anatomical 

 structure of the capillary system is to disseminate a comparatively small 

 quantity of blood over a very large space, so that the physiological 

 reactions necessary to nutrition take place with promptitude and energy. 

 Although the rate of movement of the blood in these vessels, accordingly, 

 is a slow one, yet as the distance to be passed over between the arteries 

 and veins is very small, the blood requires but a short time to traverse 

 the capillary system, and to commence its returning passage by the veins. 



General Rapidity of the Circulation. 



The rapidity with which the blood passes through the entire round 

 of the circulation has been demonstrated by Hering, Poisseuille, Mat- 

 teucci, and Vierordt in the following manner : A solution of potassium 

 ferrocyanide was injected into the right jugular vein of a horse, at the 

 same time that a ligature was placed upon the corresponding vein on 

 the left side, and an opening made in it above the ligature. The blood 

 flowing from the left jugular vein was then received in separate vessels, 

 which were changed every five seconds, and the contents afterward ex- 

 amined. It was thus found that the blood drawn from the first to the 

 twentieth second contained no traces of the ferrocyanide ; but that which 

 escaped from the vein at the end of from twenty to twenty-five seconds, 

 showed unmistakable evidence of the presence of the foreign salt. The 

 potassium ferrocyanide must, therefore, during this time, have passed 

 from the point of injection to the right side of the heart, thence to the 

 lungs and through the pulmonary circulation, to the left side of the 

 heart by the pulmonary veins, outward by the arteries to the capillary 

 circulation of the head and neck, and must have again commenced its 

 downward passage to the heart by the opposite jugular vein. 



By extending these observations, it was found that the duration of 

 the circulatory movement varies to some extent in different species of 

 animals ; being, as a general rule longer in those of larger size. The 

 main result, as given by Milne Edwards, 1 is as follows: 



1 Leqons sur la Physiologic. Paris, 1859, tome iv. p. 364. 



