224 



HITCHCOCK'S ANATOMY 



suit can not be relied on for perfect accuracy; and that tins is 

 no inconsiderable force may be inferred from the great amount 

 of resistance that is offered to this current from the ramifica- 

 tions of the smaller arteries, whereby the velocity is increased 

 as well as the surface of resistance. It may also be seen by 

 wounding an artery of medium size, when the blood is sent 

 out in jets, and sometimes to the distance of several feet. 

 This, however, is not the whole amount of the propulsive 

 force in the arterial system, for the middle or elastic coat as- 

 sists by a secondary action. When the ventricle contracts, or 

 the heart beats, the blood is driven into the aorta and its 

 larger branches with so much force, that the middle coat yields 

 considerably, and the artery is distended beyond its ordinary 

 size. As soon as the contraction of the ventricle ceases, of 

 course the blood is forced back towards the heart by the elas- 

 ticity of the artery. But as soon as it commences to flow to- 

 wards the heart the semi-lunar valves close at once, prevent- 

 ing the flow in that direction, so that the whole force of the 

 artery is expended in driving the blood towards the extremities. 



390. Pressure of the Muscles. — Another power which 

 aids in the propulsion of the arterial blood is the pressure of 

 the muscles upon the arteries. This is effected by the en- 



- largement which always takes place in the belly of the mus- 

 cle whenever it is used. This is not a constant power, acting 

 only during exercise of the muscles. 



391. Forces of the Venous Circulation. — The agencies 

 by Avhich the blood is returned to the heart are not so well 

 known as those just considered. The valves in the lining 

 membrane of the veins seem to be a contrivance to supply the 

 deficiency of power to drive the blood back. 



392. Pressure of the Muscles. — The pressure of the 

 muscles, without doubt, is another important impulse in 



When can the forces of the circulation in arteries be well seen ? How does the mid- 

 dle coat assist in this work? Of what use are the semi-lunar valves? 390. How does 

 the pressure of the muscles aid in the circulation? 391. How do the val ves aid in th# 

 venous circulation ? 392. Does muscular pressure aid in this ? 



