THE MECHANISM OF CO-ORDINATED MOVEMENTS 393 



the toxin formed by the tetanus bacillus. If a small dose of strychnine 

 be injected into a spinal frog, after a short period of heightened 

 irritability the slightest stimulus applied to the surface will cause 

 spasms, which may affect everj- muscle in the body. Pinching the 

 foot, instead of causing it to be drawn up now causes the legs, arms, 

 and back to be rigidly extended. The extension is not a co-ordinated 

 act, but is associated with strong contraction of the flexors, the final 

 position of the limbs being determined by the preponderating strength 

 of the extensor muscles. The real meaning of this condition is seen if, 

 in a spinal mammal, the extensor muscles be connected with a lever 

 and the flexor muscles cut. On exciting the flexor reflex by pricking 

 the foot,- there is instantaneous relaxation of the extensor muscles. 

 A small dose of strychnine is now given, insufficient to cause general 

 convulsions. It is now found that on pricking the foot the extensor 

 muscles respond, not with inhibition, but with a contraction. Strych- 

 nine acts by abolishing the inhibitory side of every co-ordinated act 

 and converting the process of inhibition into one of excitation. Co- 

 ordination therefore becomes an impossibility, and stimulation of any 

 spot excites contractions not only of the appropriate muscles but also 

 of the antagonists of these muscles, the direction of the resulting move- 

 ment being determined simply by the relative strength of the two sets 

 of muscles. 



The same effect is produced by tetanus toxin, and, since the action 

 of this toxin may be confined in its early stages to one limb, it is 

 possible to show the abolition of the inhibitor side of the reflexes in 

 this one limb while the limb of the other side reacts normally to the 

 stimulus. The same abolition of inhibition is found whether the 

 response be excited by stimulation of the skin or by voluntary excita- 

 tion from the cortex of the brain. Thus in the monkey, on stimulating 

 the cortex, opening of the mouth may be excited from all the spots 

 marked " <C " in the diagram, closure being only obtained from those 

 spots marked " = " (Fig. 174). Under the influence of the tetanus toxin 

 excitation of every one of the spots, whether " <C " or " =," causes 

 closure of the jaw. It is impossible for a patient under these circum- 

 stances to open his mouth, because every willed impulse for opening 

 innervates at the same time the stronger masseter muscles and effec- 

 tively closes the mouth. 



