88 THE MECHANICAL FUNCTIONS. 



texture is also highly elastic ; a property which is 

 exceedingly advantageous in the construction of 

 the frame. Not only the displacement of parts is 

 resisted by this elasticity, but, when displaced, 

 they tend to return to their natural position. This 

 property performs a more important part in the 

 mechanism of the animal than of the vegetable 

 system; as might, 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 continuous 

 sheet of a closer texture, after the surfaces of the 

 plates have been brought to cohere so as to oblite- 

 rate all the cellular interstices, and become imper- 

 vious to fluids. Though equally flexible and elastic 

 with the original texture of which it is formed, the 

 membrane has acquired, by this consolidation, 

 greater strength and tirmness ; 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 extent of their relative motions. They 

 furnish strong coverings for the investment, the 

 support, and the protection of all the important 



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

 Scarpa stretched a portion of peritoneum, (which is a very thin mem- 

 brane Hning the abdominal cavity), over a hoop, and placing weights 

 upon its surface, found that it did not give way till it was loaded 

 with fifteen pounds. 



