ANIMAL ORGANIZATION. 83 



of the animal than of the vegetable system: as mlglit, indeed, 

 have been anticipated from the more active and energetic 

 movements required by the functions of the former. 



The cellular texture, in its simple form, admits of the 

 ready transmission of fluids through it; but it is necessary, 

 on many occasions, to interpose a barrier to their passage. 

 Such barriers are provided in membranes^ which are merely 

 modifications of the same material, spread out into a con- 

 tinuous sheet of a closer texture, after the surfaces of the 

 plates have been brought to cohere so as to obliterate all the 

 cellular interstices, and become impervious to fluids. Though 

 equally flexible and elastic with the original texture of 

 which it is formed, the membrane has acquired, by this con- 

 solidation, greater strength and firmness, properties which 

 adapt it to a great number of important purposes.* 



Membranes are extensively employed to connect distant 

 organs, and often serve to determine the direction and ex- 

 tent of their relative motions. They furnish strong cover- 

 ings for the investment, the support, and the protection of 

 all the important organs of the body. What Paley has 

 termed ihe package of the organs is efiected 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, af- 

 ter 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 coverino;. Their in- 

 ner sides present every where a smooth and polished sur- 

 face, 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 sur- 

 faces that are in contact with one another, and obviates the 

 injury that would otherwise arise from friction. From this 



* With a view of ascertaining- the actual strength of membranes, Scarpa 

 stretched a portion of peritoneum, (which is a very thin membrane lining 

 the abdominal cavity,) over a hoop, and placing- weights upon its surface, 

 found it did not give way till it was loaded with fifteen pounds. 



