1818.] Dr. Philip on Stimulants and Sedatives. 373 



exhausted organs of sense ; but were the vital organs themselves 

 subject to similar exhaustion, what during such intervals would 

 preserve the life of the animal ? and by what powers would the 

 vigour of these organs be restored ? 



It has been said, indeed, that the diastole of the heart arises 

 from the stimulus of the blood exhausting its excitability in the 

 systole which is restored to it during the interval that elapses 

 between its contraction and the occurrence of that degree of 

 distention which again excites it. But a very simple experiment 

 shows the fallacy of this opinion. 



If the heart is exhausted by the stimulus of the blood, and 

 recovers its excitability during the absence of such a quantity of 

 this fluid as is capable of exciting it, it ought not to recover its 

 excitability if it is prevented from expelling any part of the blood 

 which has excited it ; for we have seen that the continued appli- 

 cation of the same stimulus which has produced exhaustion 

 cannot again excite the exhausted part, as no renewal of 

 excitability can take place while the agent which exhausted it is 

 still applied. The retina will never recover its powers under the 

 impression of the same degree of light which exhausted it. We 

 eannot recover from fatigue while the cause of our fatigue still 

 operates. But the alternate contractions and relaxations of the 

 heart still take place, as I have ascertained by repeated experi- 

 ment, although a ligature be thrown around the aorta, in conse- 

 quence of which, the heart remains uniformly gorged with blood. 

 The result of this experiment is not influenced by previously 

 destroying the sensibility of the animal by a blow on the occiput. 



If we sprinkle salt on a muscle, it does not occasion perma- 

 nent contraction followed by exhaustion, but a constant alter- 

 nation of contraction and relaxation, although the salt is never 

 removed. The state of the muscle, however, in the relaxations 

 which intervened between the contractions is evidently very 

 different from that in which it is left when the salt can no longer 

 excite any contraction in it. 



The foregoing facts seem to prove that the nervous and mus- 

 cular excitability obey different laws. While the effect of uniform 

 stimuli acting on the former is permanent excitement, followed 

 by exhaustion, the habit of the muscular fibre under the influence 

 of uniform stimuli is to act by intervals. This is probably the 

 cause why moderate excitement seems not to exhaust the mus- 

 cular fibre, while the nervous fibre suffers proportional exhaustion 

 from every degree of excitement. 



Two circumstances appear to be capable of occasionally 

 counteracting this habit and producing in the muscular fibre 

 permanent contraction, a peculiarly strong stimulus and the 

 influence of the will. It is only, however, occasionally that the 

 most powerful stimuli have this effect; and it is only for a 

 limited time that the will can produce it. After a certain time, 

 the natural tendency of the muscle to alternate contraction and 



