J12 NATURAL THEOLCaY. 



the flesh, the sides and summits loose ; hut, though loose, 

 connected by threads of a determinate length, with certain 

 small fleshy prominences adjoining. The effect of this con- 

 struction is, that when the ventricle contracts, the blood en- 

 deavoring to escape in all directions, and among other direc- 

 tions pressing upwards, gets between these membranes and 

 the sides of the passage, and thereby forces them up into 

 such a position, as that together they constitute, when raised, 

 a hollow cone — the strings before spoken of hindering them 

 from proceeding or separating further ; which cone entirely 

 occupying the passage, prevents the return of the blood into 

 the auricle. A shorter account of the matter may be this : 

 so long as the blood proceeds in its proper -course, the mem- 

 branes which compose the valve are pressed close to the side 

 of the vessel, and occasion no impediment to the circulation : 

 Vvhen 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 proportional 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 likev/ise repeat, what we before observed con- 

 cerning 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 or con- 

 figuration itself may seem to be produced by the action ol 

 the part, or by the action or pressure of adjoining parts. 

 Thus the bend and the internal smooth concavity 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 mus- 

 cles. But valves could not be so formed. Action and press- 



