550 DECLINE OF THE SYSTEM. 



datioii, begun from the earliest period of develope- 

 ment, is still advancing, and is producing in the 

 fluids greater thickness, and a reduction of their 

 total quantity ; and in the solids, a diminution in 

 the proportion of gelatin, and the conversion of 

 this element into albumen. Hence, all the textures 

 acquire increasing solidity, the cellular substance 

 becomes firmer and more condensed, and the solid 

 structures more rigid and inelastic : hence the 

 tendons and ligamentous fibres growing less flex- 

 ible, the joints lose their suppleness, and the con- 

 tractile power being also impaired, the muscles 

 act more tardily as well as more feebly, and the 

 limbs no longer retain the elastic spring of youth. 

 The bones themselves grow harder and more brit- 

 tle ; and the cartilages, the tendons, the serous 

 membranes, and the coats of the blood-vessels, 

 acquire incrustations of ossific matter, which inter- 

 fere with their uses. Thus are all the progressive 

 modifications of structure tending, slowly but in- 

 evitably, to disqualify the organs for the due per- 

 formance of their functions. 



Among the most important of the internal changes 

 consequent on the progress of age are those which 



many of its cells and vessels, the vitality of the interior wood may 

 be destroyed, and it then becomes liable to decay by the action of 

 foreign agents, yet the exterior layers of the liber still vegetate with 

 undiminished vigour; and, unless injured by causes extraneous to its 

 own system, the life of the tree will continue to be sustained for an 

 indefinite period. If, on the other hand, we were to regard each 

 separate shoot as an individual organic body, and every layer as 

 constituting a distinct generation of shoots, the older being covered 

 and enclosed in succession by the younger, the great longevity of a 

 tree would, on this hypothesis, indicate only the permanence of the 

 species, not the indefinitely protracted duration of the individual 

 plant. 



