98 OF THE VESSELS 



to the side of the vessel, and occasion no impedient to the 

 circulation ; when the blood would regurgitate, they are 

 raised from the side of the vessel, and meeting in the middle 

 of its cavity, shut up the channel. Can any one doubt 

 of contrivance here ; or is it possible to shut our eyes 

 against the proof of it ? 



This valve, also, is not more curious in its structure, 

 than it is important in its office. Upon the play of the 

 valve, even upon the proportioned length of the strings or 

 fibres which check the ascent of the membranes, depends, as 

 it should seem, nothing less than the life itself of the animal. 

 We may here likewise repeat, what we before observed 

 concerning some of the ligaments of the body, that they 

 could not be formed by any action of the parts themselves. 

 There are cases, in which, although good uses appear to 

 arise from the shape or configuration of a part, yet that 

 shape and configuration itself may seem to be produced by 

 the action of the part, or by the action or pressure of adjoin- 

 ing parts. Thus the bend, and the internal smooth con- 

 cavity of the ribs, may be attributed to the equal pressure 

 of the soft bowels ; the particular shape of some bones and 

 joints, to the traction of the annexed muscles, or to the 

 position of contiguous muscles. But valves could not be 

 so formed. Action and pressure are all against them. The 

 blood, in its proper course, has no tendency to produce 

 such things ; and, in its improper or reflected current, has 

 a tendency to prevent their production. Whilst we see 

 therefore the use and necessity of this machinery, we can 

 look to no other account of its origin or formation than the 

 intending mind of a Creator. Nor can we without admi- 

 ration reflect, that such thin membranes, such weak and 

 tender instruments, as these valves are, should be able to 

 hold out for seventy or eighty years. 



Here also we cannot consider but with gratitude, how 

 happy it is that our vital motions are involuntary. We 

 should have enough to do, if we had to keep our hearts 

 beating, and our stomachs at work. Did these things de- 

 pend, we will not say upon our eflfort, but upon our bidding, 

 our care, or our attention, they would leave us leisure for 

 nothing else. We must have been continually upon the 

 watch, and continually in fear ; nor would this constitution 

 have allowed of sleep. 



It might perhaps be expected, that an organ so precious, 

 of such central and primary importance, as the heart is, 

 ghQuld be defended by a case. The fact is, that a mem- 



