358 GENERAL PHYSIOLOGY 



place, but the phenomena of fatigue and exhaustion develop (these 

 will be discussed elsewhere) ; irritability gradually decreases, and 

 death finally results. In contrast to these phenomena both of 

 adaptation and fatigue, in a few cases wiih prolonged stimulation 

 the reactions continue with equal intensity. An example of such 

 cases is afforded hj the muscles of the mammalian body, which ex- 

 ist in a certain state of excitation, or, to use the common term, pos- 

 sess a " tone." Such are especially the muscles that close the 

 urinary bladder and the anus. These muscles are in a constant 

 state of contraction, which is caused by stimuli that come from 

 the cells of the nerv^ous system and act uninterruptedly upon the 

 former. The skeletal muscles also possess a constant, feeble tone, 

 which is maintained by feeble stimuli coming mostly from the 

 periphery and transmitted to them through the nervous system. 



With brief stimulation the reactions give place, usually soon 

 after its cessation, to the normal condition of the organism, but 



Fio. 161. — Guinea-pig, lying motionless upon his back, with the muscles of the extremities 

 tonically contracted. The legs stand out stiffly. 



there are cases in which the extinction does not begin immediatel}', 

 but a long, under some circumstances a very long, after-effect exists. 

 Thus, a single brief stimulus can put into long-continued, tonic ex- 

 citation certain ganglion-cells and the muscles innervated by them. 

 If, e.g., we seize a guinea-pig with the hands firmly but without 

 o-reat pressure, and turn him suddenly upon his back, he makes a 

 few, brief, defensive movements and then lies motionless. It can 

 be seen that the muscles of the extremities, which just before had 

 made the defensive movements, are strongly contracted, so that the 

 limbs stand out stifHy (Fig. 151 ). When the animal is undisturbed, 

 this condition of tonic excitation may continue for a half-hour. 



The phenomena of prolonged reflex tcine after brief stimulation 

 may be seen still more clearly in frogs that have been deprived 

 of their cerebrum. If such a frog sitting quietly in the customary 

 squatting attitude (Fig. 152, A) \)q gently stroked by two fingers 

 along the sides of the sjiinal column, he raises himself upon his 



