CH. v.] SYSTEM OF THE FORCES OF ANIMAL LIFE. 347 



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nimal, namely, to secrete and transmit the vital spirits (374). 



t is naturally subordinate to the animal force of the heart, and 

 continues so, firstly, so long as the blood continues to flow 

 through the vessels in virtue of the hearths action ; and secondly, 

 so long as the vessels respond to the impressions made by the 

 blood circulating through them. These two conditions are 

 alone requisite to the maintenance of this natural vital force 

 of the brain (663, 664). But when the secretion of the vital 

 spirits is arrested, or their flow to the cardiac nerves cut ofi", 

 then the hearths movements are easily interrupted, and at last 

 stop, so that when the circulation of the vital spirits in the 

 cardiac nerves ceases, (which may result from the interruption 

 of all the cerebral functions, of the connection between the heart 

 and brain, &c.,) this primary vital force of the brain ceases. 



680. All other animal forces are naturally subordinate to 

 these two primary vital forces, in so far as the operation of all 

 in the animal machines presupposes the influence of the vital 

 spirits which render the animal machines fit for their func- 

 tions, and which they cannot be, without the natural subordina- 

 tion of both j and again, in so far as every animal machine is 

 nourished and developed by the circulation of the blood, their 

 natural functions are eff'ected also by it. 



681. In especial, the natural functions of all parts of animal 

 organs are naturally subordinate to the combined action of the 

 two primary vital forces of the heart and brain, as may be 

 easily shown. This is manifest with reference to the animal 

 force of the arteries, in forwarding the general circulation of 

 the fluids, since they become fit for their function in virtue of 

 the vital spirits, and are dependent on the heart for receiving 

 the impressions which excite their stroke (460). 



682. The animal forces of the capillaries and their termina- 

 tions stand in the same natural subordination, whereby they 

 excite a flux towards parts duly irritated (207). The circula- 

 tion must supply the fluids which constitute the flux, and it 

 is through the vital spirits that they are rendered capable of 

 responding to stimuli (462, 463). But a co-operating or co- 

 ordinate force is necessary to this flux, and the subordination 

 of this animal function is also conditional, inasmuch as it only 

 results when an external impression takes place at the same 

 time on the capillaries, or their mouths (207, 462). 



