ANIMAL ORGANIZATION. 89 



organs of the body. What Paley has termed the 

 package of the organs is effected 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 membranes, 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 contained 

 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 neighbour- 

 hood of joints, closed cavities of the same descrip- 

 tion, 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 impermeable to fluids, are 

 extensively employed as receptacles for retaining 

 them ; forming, in the first place, sacs, or pouches 

 of various kinds for that purpose. The ink-bag of 

 the cuttle fish, the gall-bladder, and even the 

 stomach itself, are examples of this kind of structure. 

 The coats of these sacs, being very extensible and 

 elastic, readily accommodate themselves to the 

 variable bulk of their contents. 



