VALVES OF THE VEINS. 287 



at London Bridge ; and the water roaring in its 

 passage through that pipe is inferior in im- 

 petus and velocity to the blood gushing through 

 the whale's heart. An anatomist who under- 

 stood the structure of the heart, might say before- 

 hand that it would play ; but he would expect, 

 from the complexity of its mechanism, and the 

 delicacy of many of its parts, that it should always 

 be liable to derangement, or that it would soon 

 work itself out. Yet shall this wonderful ma- 

 chine go on, night and day, for eighty years 

 together, at the rate of a hundred thousand 

 strokes every twenty- four hours, having at every 

 stroke a great resistance to overcome, and shall 

 continue this action, for this length of time, 

 without disorder and without weariness. To 

 those who venture their lives in a ship, it has 

 often been said that there is only a plank be- 

 tween them and destruction ; but in the body, 

 and especially in the arterial system, there is 

 in many parts only a membrane, a skin, a 

 thread." Yet how well has every part been 

 guarded from injuiy : how providentially have 

 accidents been anticipated : how skilfully has 

 danger been averted ! 



The impulse which the heart, by its powerful 

 contraction, gives to the blood, is nearly ex- 

 pended by the time it has reached the veins : 

 nature has accordingly furnished them with 

 numerous valves, all opening, of course, in a 



