^4 LECTURE I. 



effect of absolute inability, in the irritable 

 property, to continue in action, for some 

 muscles continue to act without experien- 

 cing fatigue. For instance, those of the jaws 

 and back ; for whenever they relax the 

 jaw drops, and the head and body fall for- 

 wards, as we see in persons who are going 

 to sleep in a sitting posture. Certain 

 sphincter muscles likewise remain in action 

 without experiencing fatigue. Some sphinc- 

 ters also, I may add, are disposed to yield 

 considerably without impatience ; so that 

 their irritability resembles that of those 

 muscles which Bichat has considered as a 

 distinct class, and subservient alone to what 

 he calls the organic life. The contractile 

 power of muscles is also capable of remain- 

 ing in vehement action for a great length of 

 time, as we see in some cases of cramps, and 

 still more in some cases of tonic tetanus. 



Yet, though the irritable power is not 

 incapable of continued exertion, it seems 

 evidently to be in general susceptible of 

 fatigue, and inclines to be at rest. If 

 we stimulate the muscles of a limb of: a 



