820 THE CENTRAL NERVOUS SYSTEM 



to observe their behavior when an afferent nerve is stimulated which un- 

 der ordinary conditions would throw them into inhibition. Such a 

 preparation can be obtained in the hind limb of a dog by cutting all the 

 muscles that act on the knee joint except the vastus crureus, which in a 

 normal animal invariably undergoes inhibition when the central end of 

 the internal saphenous nerve is stimulated. If a suitable dose of strych- 

 nine is injected, it will be found that stimulation of the internal saphenous 

 nerve, in place of inhibition, causes contraction of the vastus crureus 

 muscle. The same result is obtained by injection of tetanus toxin. 



The failure of the reflex inhibition explains the symptoms produced 

 by these substances. It explains, for example, the well-known rigidly 

 extended condition of the limbs in strychnine poisoning, and the dis- 

 tressing symptom of lockjaw in tetanus infection. In this latter con- 

 dition the sufferer* is subjected to extreme torture with every endeavor 

 that he makes to open the jaw for the purpose of taking food or drink. 

 Firmer closure is the result because the normal inhibition of the temporal 

 and masseter muscles does not occur, but instead they become excited 

 and the jaw all the more firmly closed. Not only does the inhibition fail 

 to occur, but the above muscles are usually in a state of constant hy- 

 perexcitability, which it is impossible for the patient to restrain ; indeed, 

 whenever he attempts to do so the opposite occurs and the excitation 

 becomes heightened. Chloroform acts on reciprocal innervation in an 

 opposite way from strychnine and tetanus ; namely, it paralyzes the ex- 

 citation of the contracting muscles. 



Finally, it must be pointed out that this mechanism of reciprocal in- 

 nervation is by no means confined to the voluntary muscles. We have 

 already seen that it occurs in the case of the myenteric reflex. It is also 

 a most important function in the innervation of the blood vessels, dilata- 

 tion in one vascular area being accompanied by constriction in another. 

 These facts have been already sufficiently dwelt upon elsewhere (page 

 243). Sometimes also we may have reciprocal action between differently 

 acting nervous mechanisms, as for example in the case of the submaxil- 

 lary glands, which respond to stimulation of the chorda tympani nerve 

 by dilatation of the blood vessels, an inhibition of their tone occurring 

 along with stimulation of the activity of the gland cells. 



