142 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



A brief statement of the more or less unsatisfactory theories of 

 inhibition is given in connection with the inhibitory action of the 

 vagus nerve on the heart beat (see p. 539). It should be added, 

 however, in this connection that stimulation of the cord, and 

 probably of other parts of the nervous system, from two different 

 sources may result not only in an inhibition of the reflex normally 

 occurring from one of the stimuli, but under some circumstances 

 may give an augmentation or reinforcement of the reflex. A 

 striking example of this augmenting effect is given below in the 

 paragraph upon the knee-kick. 



Influence of the Condition of the Cord on its Reflex Ac- 

 tivities. The time and extent of the reflex responses may be 

 altered greatly by various influences, particularly by the action 

 of drugs. The effect in such cases is usually upon the nerve centers, 

 that is, upon the cells themselves or upon the connections between 

 the terminal arborization and the dendrites the process of conduc- 

 tion within the sensory and motor fibers being less easily affected. 

 A convenient method of studying such influences is that employed 

 by Tiirck. In this method the reflex frog is suspended, and the 

 tip of the longest toe is immersed to a definite point in a solution 

 of sulphuric acid of a strength of 0.1 to 0.2 per cent. If the time 

 between the immersion and the reflex withdrawal of the foot is 

 noted by a metronome, or by a record upon a kymograph, it will 

 be found to be quite constant, provided the conditions are kept 

 uniform. If the average time for this reflex is obtained from a 

 series of observations it is possible to inject various substances 

 such as strychnin, chloroform, potassium bromid, quinin, etc. 

 under the skin, and after absorption has taken place to determine 

 the effect by a new series of observations. So far as drugs are 

 concerned the results of such experiments belong rather to pharma- 

 cology than to physiology. The method in some cases brings out an 

 interesting difference in the effects of various kinds of stimulation. 

 Strychnin, for instance, as was stated above, increases greatly the 

 delicacy of the reaction to pressure stimulation. At one stage in 

 its action before the convulsive responses are obtained the threshold 

 stimulus is greatly lowered, mere contact with the toes causes a 

 rapid retraction of the leg; whereas in the normal reflex frog a 

 relatively large pressure is necessary to obtain a similar response. 

 At this stage in the action of the strychnin the effect of the acid 

 stimulus, on the contrary, may be markedly weakened so far as 

 the time element is concerned. If the action of the strychnin is 

 not too rapid, it is usually possible to find a point at which the 

 time for the reflex is diminished, but this effect quickly disappears 

 and the period between stimulus and response becomes markedly 

 lengthened at a time when the slightest mechanical stimulation gives 



