338 ANIMAL NATURE AS A WHOLE. [m. 



all these conditions cannot possibly be coincident in an isolated 

 portion of the brain, or of a nerve, since they are dependent 

 on the connection of the whole, and imply the existence of 

 the natural structure of the animal machines. 



662. Hence it follows, that although the nerves and brain 

 [the animal machines] daily receive new elements by nutrition 

 and growth, and wear away by daily use, still they can continue 

 their functions, so long as this increase or change of their 

 elements neither alters their structure, so as to render them 

 incapable of their functions, nor prevents the action of the ani- 

 mal-motor forces, for their individual constituent parts have no 

 share in these functions, except under these conditions. Indeed, 

 so far from the gradual and imperceptible growth and change, 

 which the brain and nerves undergo from continued use, ren- 

 dering them unfit for their functions, and preventing the action 

 of the animal-motor forces, they are the rather rendered more 

 and more capable of new functions (644 — 648). All plants 

 which daily lose old elements and receive new, their organic 

 functions going on uninterruptedly, are illustrations of these 

 views; the heart in animals, considered as a mere mechanical 

 machine, is another, for although the heart of the old man con- 

 tains not one of the constituents which made up the heart of 

 the child, the general change of all its constituent parts has in 

 no degree interfered with its functions, they having gone 

 on without a moment^s interruption. Thus also it is with the 

 brain and nerves. 



663. The structure of the brain and nerves renders them 

 capable of their proper and natural functions, and is necessary 

 to animal life in general, but it does not excite those functions. 

 This is the case with all machines, whether mechanical, organic, 

 or animal. A watch or a mill does not go merely because its 

 machinery is complete in all its parts, but the action of the 

 motor force appropriate to the machinery is requisite. The 

 whole body of the animal is so constructed in its perfect state, 

 that it can perform all the movements to which nature has des- 

 tined it, but it is endowed with true animal life, solely in virtue.: 

 of the animal forces which put it into motion ; and so soon as 

 these cease to act, animal life ceases, although its nervous struc- 

 ture has undergone no change whatever (638). 



664. There may be also such a condition of the animal, that 



