86 THE MECHANICAL FUNCTIONS. 



city. 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 fibrous 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 pres- 

 sure. In other cases these sheets of fibrous membrane are 

 employed as bandages, tightly bracing the muscles, and re- 

 taining them in their relative situations. The joints are sur- 

 rounded by similar bandages, known by the name of Cap- 

 sular Ligaments. 



In following the series of animal structures in the order 

 of their increasing density, we find the proportion of albu- 

 men which enters into their composition becoming greater, 

 while that of the gelatin and mucilage diminishes. When 

 the product is more uniform in its composition it is in gene- 

 ral less elastic than when it consists of a more complex com- 

 bination 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 inextensible cords, constituting the ordinary Liga- 

 vients, and the Tendons. These structures resist with 

 great power any force calculated to extend them: a proper- 

 ty which of course excludes elasticity, but, when united 

 with flexibility, implies great toughness. In a word, they 

 possess all the qualities that can be desired in a rope. It 

 will hardly be credited how great a force is required to 

 stretch, or rather rend asunder a ligament; for it will not 

 yield in any sensible degree until the force is increased so 

 enormously as at once to dissever the whole contexture of 

 its fibres. Nothing can be more artificially contrived than 

 the interweaving of the fibres of ligaments; for they are not 

 only disposed, as in a rope, in bundles placed side by side, 

 and apparently parallel to each other: but, on careful exami- 



