206 THE VITAL FUNCTIONS. 



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 between 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 provi- 

 dentially 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, of course, 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 

 255 retrograde motion is effectually prevented. 



Hence external pressure, occasionally ap- 

 plied to the veins, assists in promoting the 

 flow of blood towards the heart; and hence 

 the effects of exercise in accelerating the cir- 

 culation. Valves are more especially pro- 

 vided in the veins which pass over the mus- 

 cles of the extremities, or which run imme- 

 diately beneath the skin; while they are not 

 found in the more internal veins belonging to 

 the viscera, which are less exposed to une- 

 qual pressure. These valves are delineated in Fig. 365, 

 which represents the interior of one of the larger veins. 



The situation and structure of the valves belonging to 

 the hydraulic apparatus of the circulation furnish as une- 

 quivocal proofs of design as any that can be adduced. It 

 was the observation of these valves that first suggested to 

 the mind of Harvey the train of reflections which led him 

 to the discovery of the real course of the blood in the veins, 

 the arteries and the heart. This great discovery was one 

 of the earliest fruits of the active and rational spirit of in- 

 quiry, which, at the era of Bacon's writings, was beginning 

 to awaken the human mind from its long night of slumber, 



