General Principles of Pht/siology. 263 



were made, not on the living, but newly dead animal, which was 

 always employed if the nature of the experiment admitted 

 of it*. 



It appeared, from repeated experiments, that neither chemical 

 nor mechanical agents, applied to the brain and spinal marrow, 

 affect the action of the heart, unless they make their impression 

 on a large portion of these organs. Every part of them may be 

 stimulated individually, without the action of the heart being 

 influenced; and the agent being the same, its influence on this 

 organ is always proportioned to the extent of surface to which 

 it is appliedt. It does not appear that it is of much importance 

 on what part of the brain the agent makes its impression, Even 

 stimulating the surface alone, either mechanically or chemically, 

 immediately increases the action of the heart. 



Another circumstance, which appears to be of great conse- 

 quence in explaining the difference of the effects of agents ap- 

 plied to the brain and spinal marrow on the two sets of muscles, 

 IS, that the heart obeys a much less powerful stimulus than the 

 muscles of voluntary motion do. The most powerful chemical 

 agents alone affected them, while all that were tried readily 

 influenced the action of the heart X. Mechanical agents which, 

 by bruising and dividing the parts, occasion the greatest pos- 

 sible irritation, are best fitted to excite the muscles of voluntary 

 motion. Chemical agents, indeed, from their eftects on the 

 heart, we should, at first view, consider the most powerful. But 

 their greater effect on this organ is readily explained by what 

 has just been said. When the effect of the mechanical agent 

 was rendered extreme and general, its influence on the heart 

 was found much greater than that of any chemical agent which 

 was tried. We have seen that suddenly crushing any con- 

 siderable portion of the brain or spinal marrow instantly de- 

 stroys the function of that organ. 



The conclusions, then, at which we arrive are, that the heart 



* Why the newly dead animal is as good a subject for many physiological 

 experiments as the living one, will appear from what I shall afterwards 

 have occasion to lay before the reader. 



t E\jjci. Inq. Exper. 4 1 , 1'.', 43, J lb. Exper, 35, 30', 37. 



