90 



THE MECHANICAL FUNCTIONS. 



In the second place, we find membranes com- 

 posing tubes of various descriptions for conducting 

 fluids. Thus, in the higher classes of animals, the 

 whole of the body is traversed by innumerable 

 canals, conveying different kinds of fluids. These 

 canals, when uniting into trunks, or subdividing 

 into branches, are called Vessels (Fig. 26.) 



The fluids contained in vessels are never stagnant, 

 but are almost always carried forwards in one con- 

 stant direction. For preventing the retrograde 

 motions of the fluids passing along these canals, re- 

 course is had to the beautiful contrivance of valves. 

 The inner membrane of the vessel is employed to 

 construct these valves ; for which purpose it is ex- 

 tended into a fold, having the shape of a crescent, 

 and fixed by its convex edge to the sides of the 

 vessel, while the other edge floats loosely in its 

 cavity. Whenever the fluid is impelled in a direc- 

 tion contrary to its proper course, it raises the 

 loose edge of the valve, which, being applied to 

 the opposite side of the canal, eflectually closes the 

 passage. On the contrary, it presents no obstacle 

 to the natural flow of the contents of the vessel, 

 both edges being then closely applied to the same 

 side. Frequently two, or even three valves are used 

 at the same part, their edges being made to meet 

 in the middle of the passage, like the floodgates, or 



