350 NATURAL THEOLOGY. 



practice prove to us why the united areas of the 

 smaller branches of the artery are larger than 

 that of the trunk from which they are derived ? 

 If any further explanation be necessary, it is this 

 — that the water flowing in a tube runs more ra- 

 pidly in the centre than at the sides — or, in other 

 words, that there is a certain attraction or friction 

 at the sides. We see this in standing by a flow- 

 ing river; the friction of the water against the 

 bottom and the sides retards the stream, whilst 

 the velocity of the current is the greatest in the 

 middle. As the water in the river is delayed at 

 the bottom and sides, so is the fluid nearest the 

 sides of the tube retarded by the friction between 

 the fluid and the solid. Thus we see a remark- 

 able coincidence between the increasing diameters 

 of the circulating vessels, and of tubes laid down 

 upon accurate hydraulic principles. 



From the diameter of the arteries being larger 

 as they recede from the heart, two advantages are 

 obtained ; first, that the blood is driven on with 

 greater ease ; and secondly, that the extreme arte- 

 ries become in some measure like the veins, reser- 

 voirs of blood. A man of middling stature has 

 thirty-three pounds of blood in his circulating 

 vessels ; and did the vessels not enlarge as they 

 receded from the heart, there would be no place 

 for the deposit of this great quantity of blood. 



We may venture upon some further illustra- 

 tions. — A stream of water, unconfined, will take 

 a very different form if falling by its own gravity 



