172 NATURAL THEOLOGY. 



closing it, whenever the blood, in consequence of 

 the relaxation of the ventricle, would attempt to 

 flow back. There is some variety in the con- 

 struction of these valves, though all the valves of 

 the body act nearly upon the same principle, and 

 are destined to the same use. In general they 

 consist of a thin membrane, lying close to the side 

 of the vessel, and consequently allowing an open 

 passage while the stream runs one way, but thrust 

 out from the side by the fluid getting behind it, 

 and opposing the passage of the blood, when it 

 w^ould flow the other way. Where more than one 

 membrane is employed, the difterent membranes 

 only compose one valve. Their joint action fulfils 

 the office of a valve : for instance ; over the en- 

 trance of the right auricle of the heart into the 

 right ventricle, three of these skins or membranes 

 are fixed, of a triangular figure, the bases of the 

 triangles fastened to the flesh ; the sides and 

 summits loose ; but, though loose, connected by 

 threads of a determinate length, with certain 

 small fleshy prominences adjoining. The effect 

 of this construction is, that, when the ventricle 

 contracts, the blood endeavouring to escape in all 

 dii'ections, and amongst other directions 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 consti- 

 tute, when raised, a hollow cone, (the strings be- 

 fore spoken of hindering them from proceeding 

 or separating further ;) which cone, entirely occu- 



