288 THE VITAL FUNCTIONS. 



direction towards the heart ; so that, as long as 

 the blood proceeds in its natural course, it meets 

 with no impediment ; while a retrograde motion 

 is effectually prevented. Hence external pres- 

 sure, occasionally applied to the veins, assists in 

 promoting the flow of blood to- 

 wards the heart ; and hence the 

 effects of exercise in accelerating 

 the circulation. Valves are more 

 especially provided in the veins 

 which pass over the muscles 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 unequal 

 pressure. These valves are delineated in Fig. 

 365, which represents the interior of one of the 

 large veins. 



The situation and structure of the valves be- 

 longing to the hydraulic apparatus of the circu- 

 lation furnish as unequivocal proofs of design as 

 any that can be adduced. It was the observa- 

 tion of these valves that first suggested to the 

 mind of Harvey the train of reflexions 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 inquiry, 

 which at the era of Bacon's writings, was be- 



