160 NATURAL THEOLOGY. 



secondly, by these branches into trunks ; and thus 

 returns the blood (almost exactly inverting the 

 order in which it went out) to the fountain whence 

 its motion proceeded. All which is evident me- 

 chanism. 



The body, therefore, contains two systems of 

 blood-vessels, arteries and veins. Between the con- 

 stitution of the systems there are also two differ- 

 ences, suited to the functions which the systems 

 have to execute. The blood, in going out, passing 

 always from wider into naiTOwer tubes; and, in 

 coming back, from narrower into wider, it is evident 

 that the impulse and pressure upon the sides of the 

 blood-vessel will be much greater in one case than 

 the other. Accordingly, the arteries which carry 

 out the blood are formed of much tougher and 

 stronger coats than the veins which bring it back. 

 That is one difference : the other is still more arti- 

 ficial, or, if I may so speak, indicates still more 

 clearly the care and anxiety of the artificer. For- 

 asmuch as, in the arteries, by reason of the greater 

 force with which the blood is urged along them, a 

 wound or rupture would be more dangerous than 

 in the veins, these vessels are defended from injury, 

 not only by their texture, but by their situation, and 

 by every advantage of situation which can be given 

 to them. They are buried sinuses, or they creep 

 along grooves made for them in the bones ; for in- 

 stance, the under edge of the ribs is sloped and fur- 

 rowed solely for the passage of these vessels. Some- 

 times they proceed in channels, protected by stout 



