666 



AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



a voluntary muscular contraction, or an extra sensory stimulus occurring about 

 the same time that the tendon was struck, had the effect of increasing the height 

 of the kick. This was studied in detail by Bowditch and Warren, 1 and they 

 were able with great exactness to measure the interval between the contraction 

 of the muscle used for reinforcement and the time at which the tendon was 

 struck. The curve shown in Fig. 173 represents the results of these experi- 



40- 

 30- 

 20- 

 10- 







10- 



20- 



30- 



MM 



Normal. 



Time. 



OTOJT 0.4" 



07" 



1.7" 



FIG. 173. Showing in millimeters the amount by which the " reinforced " knee-kick varied from the 

 normal, the level of which is represented by the horizontal line at 0, " normal." The time intervals 

 elapsing between the clenching of the hand (which constituted the reinforcement) and the tap on the 

 tendon are marked below. The reinforcement is greatest when the two events are nearly simultaneous. 

 At an interval of 0.4" it amounts to nothing; during the next 0.6" the height of the kick is actually 

 diminished the longer the interval, after which the negative reinforcement tends to disappear; and 

 when 1.7" is allowed to elapse the height of the kick ceases to be affected by the clenching of the hand 

 (Bowditch and Warren). 



ments. It indicates that in general the closer together these two stimuli occur, 

 the greater the reinforcement. At an interval of 0.4 second no effect is pro- 

 duced by the muscular contraction. Increasing the interval only very slightly 

 has, however, the effect of greatly diminishing the height of the knee-kick 

 i. e. decreasing the strength of the discharge of the efferent cells and this 

 effect is not lost until the interval is increased to 1.7 second, when the volun- 

 tary muscular contraction ceases to modify the response. A given efferent cell 

 is thus modified in its discharge according to the several stimuli that act upon it. 

 Effects of Disuse. Studies on inactivity show that a certain amount of 

 exercise in any given cell is necessary for its proper nutrition, and if the exci- 

 tation fall below the point which causes this, the responsiveness of the cell is 

 diminished. 



For example, a strychnized reflex frog on being dipped into a solution of 

 cocaine loses in so large a measure its irritability that its responsiveness falls 

 far below that of a normal frog. 2 In this case the central system is deprived 

 by the action of the cocaine of the impulses which even in the absence of any 

 special form of irritation normally arrive from the skin, and the abolition of 

 these impulses causes a diminution in central responsiveness. Effects which 



1 Journal of Physiology, 1890, vol. xi. 



2 Poulsson : Archiv fur Pathologic und experimentdle Pharmakologie, 1885, Bd. xxvi. 



