5 
of the Heart and Arteries. 
pounds. It is probable that this calculation approaches suffi- 
ciently near to the truth : for the whole quantity of blood in 
the body being about 40 pounds, although some have sup- 
posed it only 20, others no less than 100, there is reason to 
believe that half of ; this quantity is contained in the veins of 
the general circulation, and that the other half is divided, 
nearly in equal proportions, between the pulmonary system 
and the remaining arteries of the body, so that the arteries of 
the general circulation may contain about 9 or 10 pounds. 
Haller allows 50 pounds of circulating fluid, partly serous, 
and partly red, and supposes \ of this to be contained in all 
the arteries taken together : but in a determination which 
must be in great measure conjectural we cannot expect perfect 
accuracy: and according to Haller's own account of the pro- 
portions of the sections of the arteries and veins, the large 
trunks of the veins appear to be little more than twice as capa- 
cious as those of the arteries, and the smaller branches much 
more nearly equal, so that we cannot attribute to the arterial 
system less than of the whole blood. 
It may be supposed that the heart throws out, at each pul- 
sation, that is about seventy five times in a minute, an ounce 
and a half of blood : hence the mean velocity in the aorta be- 
comes eight inches and a half in a second : and the velocity 
in each of the succeeding segments must of course be smaller, 
in proportion as the joint areas of all the corresponding sec- 
tions are larger than the area of the aorta : for example, in 
the last order of vessels, of which the diameter is the eleven 
hundredth of an inch, the velocity will be one ninety third of 
an inch : and this result agrees sufficiently well with Hales's 
observation of the velocity in the capillary arteries of a frog, 
