104 THE MECHANICAL FUNCTION. 



vessel is employed to construct these valves ; for 

 which purpose it is extended into a fold having 

 the shape of a crescent ; 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 direction contrary to its 

 proper course, it raises the loose edge of the 

 valve, which, being applied to the opposite side 

 of the canal, effectually closes the passage. On 

 the contrary, it presents no obstacle to the na- 

 tural 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 locks of a canal.* Among the 

 numberless instances of express contrivance 

 which are met with in the examination of the 

 fabric of animals, there is perhaps none more 

 striking and more palpable, than this admirable 

 mechanism of the valves. 



As we ascend from the simpler to the more 

 complicated systems of organization, adapted 

 to a greater range of faculties, we find greater 

 diversity in the mechanical means employed 



* Fig. 27, representing the section of a vessel, is intended to 

 show the position of the valves when applied to the sides of the 

 vessel, by the stream moving onwards in the direction pointed out 

 by the arrow. In Fig. 28, they are seen closing the passage 

 by the retrograde pressure of the current. 



