FORCE OF THE HEART. '261 



this is when we come to very large animals. The 

 aorta of a whale is larger in the bore than the main 

 pipe of the water-works at London Bridge ; and the 

 water roaring in its passage through that pipe is in- 

 ferior in impetus and velocity to the blood gushing 

 through the whale's heart. An anatomist who 

 understood the structure of the heart, might say 

 beforehand that it would play ; but he would ex- 

 pect, 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 machine 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 weari- 

 ness. 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 injury: 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 expended 

 by the time it has reached the veins : nature has 

 accordingly furnished them with numerous valves, 

 all opening in a direction towards the heart; so 

 that, as long as the blood proceeds in its natural 

 course, it meets with no impediment ; while a retro- 

 grade motion is effectually prevented. Hence ex- 



