102 THE MECHANICAL FUNCTIONS. 



They furnish strong coverings for the invest- 

 ment, the support, and the protection of all the 

 important organs of the body. What Paley 

 has termed the package of the organs is eiBfected 

 principally by their intervention. Membranes 

 are also employed to line the interior of all the 

 large cavities of the body, as those of the chest, 

 and of the abdomen, or lower part of the trunk 

 containing the organs of digestion. These mem- 

 branes, after lining the sides of their respective 

 cavities, are reflected back upon the organs 

 which are enclosed in those cavities, so as to 

 furnish them with an external covering. Their 

 inner sides present every where a smooth and 

 polished surface, over which the organs con- 

 tained in the cavity may glide without injury. 

 In all these cases, a thin fluid, called serum, 

 is provided, which moistens and lubricates the 

 surfaces that are in contact with one another, 

 and obviates the injury that would otherwise 

 arise from friction. From this circumstance, 

 the linings of these cavities have been termed 

 serous membranes. In the neighbourhood of 

 joints, closed cavities of the same description, 

 but of smaller size, are met with, 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 impeimeable to fluids, are 



