101 



greater ; and th great artery must always be the first 

 vessel that empties. 



The only certain way of calculating is, to find what 

 proportion the cavities of tbe vessels, of which the 

 whole body is composed, bear to the thickness of the 

 coats. This in the veins and arteries may be exactly 

 found ; but in the other vessels we onty know the 

 quantity of fluid they contain by carefully evaporating 

 as much as possible. Thus the Doctor found the fluids 

 to be in the arteries as 17 to 1 ; in the veins as 15,6 to 

 1 ; in the bones as 1 to 1. The least of these propor- 

 tions shews the liquors to be one half of the weight of 

 the body ; and if a calculation be made on the propor- 

 tion of the blood in the arteries, also to their coats in a 

 body weighing 160 pounds, there will be found 100 

 pounds of blood or circulating fluid. 



In a foetus the circulation is performed in a peculiar 

 manner. The septum, which separates the two ven- 

 tricles of the heart is pierced through with an aperture 

 called the foramen ovale, and the pulmonary artery, <i 

 Jittle after it has left the heart, sends out a tube into 

 the descending aorta, called the communicating canal. 

 When the foetus is born, the foramen ovale closes, and 

 that* canal dries up into a simple ligament. * 



The foetus while in the womb receives little air. Its 

 lungs, therefore, cannot swell and subside, they con- 

 tinue almost at rest ; nor can they allow the blood to 

 circulate either in abundance or with ease. Nature, 

 therefore, has excused them from the passage of the 

 greatest part of the blood, and has contrived the fora* 

 men ovale, by which part of the blood of the vena 

 cava passes through the right ventricle into the left, 

 and by this means it is found as far on its journey as if 

 it had passed tho lungs : but this is not all, for that 

 blood of the cava which, missing the foramen ovale, 

 passes from the right auricle into the right ventricle, 

 being still too much to pass by the lungs, the commu- 

 nicant canal intercepts part of it, and pours it imme- 

 diately into the descending aorta. 



