CHAPTER V. - 



THE SYSTEM OF THE FORCES OF ANIMAL LIFE. 



659. The mode and method by ,whicli, in the perfect con- 

 dition of animal life, tlie animal forces act with and by each 

 other, and are co-ordinate and subordinate, is termed the system 

 of the forces of animal life. 



660. The order in which a watchmaker constructs a watch, 

 when at one time he forms a wheel, at another a chain, now the 

 hands, the face, and the spring, and then connects these separate 

 pieces, and inserts the spring, cannot enable a spectator to dis- 

 cover how each of them will contribute its share to the perform- 

 ance of the whole machine, although well known to the master- 

 workman, who has worked according to a previous plan. But 

 when the watch is put together, wound up, and set in motion ; 

 in short, when the mechanism is completed and finished, the 

 spectator can understand how all the separate pieces and the 

 motor powers co-operate with each other, so as to attain the 

 intent of the machine, namely, to divide the time accurately. 

 Hitherto, we have seen how the machines and forces of the 

 animal are more and more developed, as it approaches the period 

 of maturity. It is at that period that the complete machine 

 can be investigated in all its relations, and the connection 

 of all its machinery and forces, as a system, be comprehended. 



661. The animal functions of animals are effected generally 

 by means of the nervous system and the vital spirits it contains. 

 These do not constitute the only parts which develop animal 

 movements, but in virtue of their relation, are the first that are 

 capable of them. Then all animal functions take place in virtue 

 of impressions on the animal machines (356). It is necessary 

 to the reception of an internal or external impression by a 

 nerve, that it be communicated to it in a certain direction, that 

 it be impressed on the medulla of a nerve, and be propagated 

 therein by means of the vital spirits (32, 121) ; and if the 

 brain is to receive impressions from conceptions or other causes 

 of irritation, they must take place in virtue of a current 

 [Antrieb] of the vital spirits in its minute tubes (28, 121). But 



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