84 



THE MECHANICAL FUNCTIONS. 



circumstance, the linings of these cavities have been termed 

 serous mevibranes. In the neighbourhood of joints, closed 

 cavities of the same description, but of smaller size, are met 

 v^^ith, for the obvious purpose of facilitating motion; and 

 here also friction is prevented by a highly lubricating fluid, 

 termed synovia, which is poured out between the surfaces 

 of the membrane lining the cavities. 



Membranes being impermeable to fluids, are extensively 

 employed as receptacles for retaining them: forming, in the 

 first place, sacs, or pouches of various kinds for that pur- 

 pose. The ink-bag of the cuttle fish, the gall-bladder, and 

 even the stomach itself, are examples of this kind of struc- 

 ture. The coats of these sacs, being very extensible and 

 elastic, readily accommodate themselves to the variable bulk 

 of their contents. 



In the second place, we find membranes composing 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 constant direction. 

 For preventing the retrograde motions of the fluids passing 

 along these canals, recourse is had to the beautiful con- 

 trivance of valves. The inner membrane of the 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 im- 



