148 THE BLOOD AND ITS CIRCULATION. 



a second in the caiotid artery, and Om. 44 in the aorta: it is 

 also four hundred times greater in the latter vessel than in 

 the capillaries. Similar results have been obtained with 

 the hemodromometer of Chauveau, and the hemodromo- 

 grapher of Lortet (Fig. 47), which are constructed on the 

 same principle as Vierordt's instrument. 



By means of the above data on the velocity of the blood 

 we can calculate the dimensions of the arterial cone. Indeed, 

 the velocity at the different points of the cone are in inverse 

 ratio to the surface of the section of the cone at this point : the 

 total containing space of the capillary system is thus to that of 

 the 'aorta as 400 to 1. We may therefore conclude that the 

 containing space of the aorta having a diameter of 3 centi- 

 metres, the diameter of the base of the arterial cone must 

 be about Om. 66. If the exact capacity of this cone were 

 known, it would be easy to calculate its height. These cal- 

 culations, however, yield only approximative results, for the 

 slackening of the current of the blood at the level of the 

 capillaries is also an important feature, which is not here 

 taken into account : it is caused by the net-like arrangement 

 of long and narrow tubes (see the two laws of Poiseuille, 

 p. 147, above). 



It may still be asked, after determining the velocity of the 

 blood in certain points, what is the general speed, considering 

 the circulation as a whole? In one word, how much time is 

 necessary for a blood globule to pass from the left ventricle 

 to the right auricle ? The average quantity of blood thrown 

 into the aorta at each contraction of the heart is 180 grammes. 

 As the total mass of the blood is only 5 kilogrammes, 25 or 

 30 cardiac pulsations are necessary to enable all the blood to 

 pass through the central organ, and rather more than 30 

 seconds for the return of a globule which has left the heart. 

 The result of this calculation can only be general and ap- 

 proximative; for the blood which goes to the lower limbs 

 has a much longer passage than that which passes into the 

 cardiac veins and arteries: the time of the complete journey 

 (going and returning) of a blood globule must therefore vary, 

 according to the part to which it is sent. Still, the circula- 

 tion must always be extremely rapid, as is proved by experi- 

 ment in cases of poisoning; for we know that a drop of 

 prussic acid upon the conjunctiva will kill an animal in eight 

 or ten seconds, and that the poison is found to be diffused 

 through the whole system. If the poison is placed further 

 from the heart, upon a wound in the foot, for instance, death 



