CIRCULATION OF THE BLOOD. 



43 



the left ventricle, so that, pressed by the elastic force of the 

 arterial parietes, it is continually pushed forward, from the heart 

 towards the extremities of the arteries. 



53. The phenomenon known under the name of the pulse, is 

 nothing else than the motion caused by the pressure of the blood 

 against the parietes of the arteries, every time that the heart con- 

 tracts According to the frequency and force of these motions, 

 we may judge of the manner in which the organ beats, and draw 

 therefrom, deductions useful in medicine. But the pulse is not 

 frit in all parts; to perceive it, we 



must slightly compress an artery 

 of a certain volume between the 

 finger and a resisting sut face, of 

 a bone, for example, and select a 

 vessel situated near the skin, as 

 tl e radial artery at the wrist. 



54. The impulsion received by 

 the blood at its exit from the left 

 ventricle of the heart, is commu- 

 nicated to the capillary vessels 

 and to the veins, and determines 

 the progression of the blood in 

 them. But the return of the 

 vrnous blood towards the right 

 ventricle, is favoured by some 

 other circumstances. In the veins 

 ol the extremities, the membrane 

 which lines these vessels, forms a 



pi eat many folds or vuhw, ( Fig. 10, .) which open when the blood 

 pushes them from the extremities towards the heart, and shut so 

 as to close the passage, when this liquid flows in a contrary direc- 

 ti >n. Now, this arragement prevents the blood from flowing back 

 t( -wards the capillaries, and thus facilitates its passage towards 

 the heart; for, every time a vein is pressed by the movements of 

 11 ie parts in its vicinity, the blood is pushed forward. 



Explanation oj Fig. 10. A large vein opened to show the valves, (e,) 



which are found in these vessels, 0,0, openings of two of its branches, an, 



anastomosis of two veins, c, capillary roots of the vein. 



53. What is the pulse ? Is the pulse ftlt in all parts ? What does the pulse 

 indicate ? What oiicumstanccs are to be observed in order to feel the pulse ? 



54. Whftl causes the blood to circulate in the capillary vessels? What 

 other circumstance, besides the impulsion received from the heart, favour* 

 Hie return of the blood towards the right ventricle ? 



