8 THE VITAL FUNCTIONS. 



possess an extensive system of circulating and ab- 

 sorbing vessels, the changes which are effected are 

 so considerable and so rapid, that even in the 

 densest textures, such as the bones, scarcely any 

 portion of the substance which originally composed 

 them is permanently retained in their structure. 

 To so great an extent is this renovation of materials 

 carried on in the human system, that doubts may 

 very reasonably be entertained as to the identity 

 of any portion of the body after the lapse of a cer- 

 tain time. The period assigned by the ancients 

 for this entire change of the substance of the body 

 was seven or eight years ; but modern inquiries, 

 M Inch show us the rapid reparation that takes place 

 in injured parts, and the quick renewal of the bones 

 themselves, tend to prove that even a shorter time 

 than this is adequate to the complete renovation 

 of every portion of the living fabric* 



Imperfect as is our knowledge of organic che- 

 mistry, we see enough to convince us that a series 

 of the most refined and artificial operations is re- 

 quired in order to bring about the complicated and 

 elaborate arrangements of elements which consti- 

 tute both animal and vegetable products. Thus in 

 the very outset of this, as of every other inquiry in 

 Physiology, we meet with evidences of profound 

 intention and consummate art, infinitely surpassing 

 not only the power and resources, but even the 

 imagination of man. 



Much as the elaborate and harmonious me- 

 chanism of an animal body is fitted to excite our 

 admiration, there can be no doubt that a more ex- 



* See the article " Ac e" in the Cyclopsedia of" Practical Medicine, 

 where I have enlarged upon this subject. 



