OF ANIMAL BODIES. 97 



can. When the piston descends in a pump, if it were not 

 for the stoppage by the valve beneath, the motion would 

 only thrust down the water, which it had before drawn up. 

 A similar consequence would frustrate the action of the 

 heart. Valves, therefore, properly disposed, i. e. properly 

 with respect to the course of the blood which it is neces- 

 sary to promote, are essential to the contrivance. And 

 valv&s so disposed a?'e, accordingly, protnded. A valve \s 

 placed in the communication between each auricle and its 

 ventricle, lest when the venticle contracts, part of the blood 

 should get back again into the auricle, instead of the whole 

 entering, as it ought to do, the mouth of the artery. A valve 

 is also fixed at the mouth of each of the great arteries which 

 take the blood from the heart ; leaving the passage free, so 

 long as the blood holds its proper course forward ; closing 

 it, whenevei the blood, in consequence of the relaxation of 

 the ventricle, would attempt to flow back. There is some 

 variety in the construction 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 whilst the stream 

 runs one way, but thrust out from the side by the fluid get- 

 ting behind it, and opposing the passage of the blood,, 

 when it would flow the other way. Where more than one 

 membrane is employed, the different membranes only com- 

 pose one valve. Their joint action fulfils the office of a 

 valve : for instance ; over the entrance 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 sum- 

 mits 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 directions, and amongst other directions pressing up- 

 wards, 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 in- 

 to the auricle. A shorter "account of the matter may be 

 this. So long as the blood proceeds in its proper course, 

 the membranes which compose the valve are pressed close 



