VESSELS OF ANIMALS. Ill 



the course intended, but back from the artery, through which 

 it ought to be moving forward. The way of preventing a 

 reflux of the fluid in both these cases, is to fix valves, which, 

 like floodgates, may open a way to the stream in one direc- 

 tion, and shut up the passage against it in another. The 

 heart, constituted as it is, can no more work without valves 

 ihan a pump can. When the piston descends in a pump, 

 if it were not for the stoppage by the valve beneath, the mo- 

 tion would only thrust down the water which it had before 

 drawn up. A similar consequence would frustrate the ac- 

 tion of the heart. Valves therefore, properly disposed, that 

 is, properly with respect to the course of the blood w^hich it 

 is necessary to promote, are essential to the contrivance ; 

 and 'calves so disjyosed are accordingly provided. A valve 

 is placed in the communication between each auricle and its 

 ventricle, lest, when the ventricle 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 

 whenever the blood, m consequence of the relaxation of the 

 ventricle, would attempt to flow back. There is some vari- 

 ety in the construction of these valves, though all the valves 

 of the body act nearly upon the same principle, and are des- 

 tined to the same use. In general they consist of a thin 

 membrane, lying close to the side of the vessel, and conse- 

 quently 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 would flow 

 the other way. Where more than one membrane is em- 

 ployed, the different membranes only compose 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 



