92 THE MECHANICAL FUNCTIONS. 



extension, they are necessarily devoid of elasticity. 

 Hence they are adapted to form external tunics for 

 the investment of such organs as are not intended 

 to vary in their size. Occasionally these Jibroiis 

 capsules, as they are called, send down processes 

 into the interior of those organs, for the purpose of 

 giving them mechanical support. This is the case, 

 for instance, with the membranes surrounding the 

 brain of quadrupeds, and which form two partitions, 

 the one vertical, the other horizontal ; both being 

 firmly stretched in their respective positions, and 

 serving to divide the pressure. In other cases 

 these sheets of fibrous membrane are employed as 

 bandages, tightly bracing the muscles, and retain- 

 ing them in their relative situations. The joints 

 are surrounded by similar bandages, known by the 

 name of Capsular Ligaments. 



In following the series of animal structures in 

 the order of their increasing density, we find the 

 proportion of albumen which enters into their com- 

 position becoming greater, while that of the gelatin 

 and mucilage diminishes. When the product is 

 more uniform in its composition, it is in general 

 less elastic than when it consists of a more complex 

 combination of ingredients. A great preponderance 

 of albumen tends also to diminish the elasticity. 

 Thus the densest kinds of fibrous texture present, 

 instead of thin and broad expansions of elastic 

 membrane, the thick and elongated form of inex- 

 tensible cords, constituting the ordinary Ligaments, 

 and the Tendons. These structures resist with 

 great power any force calculated to extend them ; 

 a property which of course excludes elasticity, but, 

 when united with flexibility, implies great tough- 



