MOTION OF THE BLOOD IN THE ARTERIES. 89 



seated far beneath the muscles, and close to the bones, 

 and are never found in places where they would be ex- 

 posed to injuries. Thus, no large arteries are found on 

 the back or face ; those of the chest run along on the in- 

 ner side of the ribs, and those of the legs where accidents 

 are frequently occuring, are very deeply seated near the 

 bones and are difficult of access. 



Emily. But, Dr. B., are the arteries entirely passive, 

 as it respects the motion of the blood ? As the heart, 

 according to your definition, is a power whose object is 

 merely to assist the motion of the blood, I still presumed 

 that they had some share in the duty. 



Dr. B. The motion of the ulood in the larger ar- 

 teries is derived entirely from the impulse of the heart, 

 but in the smaller branches it is chiefly effected by the 

 action of the vessels themselves. It may be stated gen- 

 erally, that as we proceed from the heart we find its 

 power diminishing, and that of the arteries increasing, 

 until we arrive at the extremities of the vessels where 

 the force of the heart is nothing. Though the blood is 

 thrown into the arteries at intervals, yet they are always 

 full the cause of this, I suppose, 1 need not point out 

 to you. 



Emily. If I am not mistaken it is because a column 

 of blood being displaced, no greater than that which en- 

 ters the vessels at each contraction of the ventricle, they 

 will be equally full during both the contraction and dila- 

 tion of the heart. But will the motion of the blood be 

 uniform ? 



Dr. B. Certainly not, for during the dilatation of the 

 heart, its progress must unavoidably be slackened, and if 

 a puncture be made in an artery the blood will gush out 

 in regularly successive jets. If the motion of the blood 

 were uniform, it would flow out in a steady stream. This 

 flowing of successive waves of blood may be easily per- 

 ceived by placing the finger over an artery, and pressing 

 upon it with sufficient force to diminish its calibre slight- 

 ly. This presents an obstacle to the free passage of the 

 8* 



