CHAP, i.] THE SPINAL CORD. 927 



be increased or diminished, augmented or inhibited by a coincident 

 voluntary effort directed towards some other end, or by the coinci- 

 dent development of a sufficiently distinct sensation. In general 

 it may be said that whatever favours the activity of the spinal 

 cord tends to increase them, and whatever depresses the activity 

 of the spinal cord tends to diminish them. They are diminished 

 or wanting in certain diseases of the spinal cord (e.g. locomotor 

 ataxy), and exaggerated in others ; so much so indeed that they 

 have become of practical clinical importance as a means of 

 diagnosis. Whether we regard them as instances of ordinary 

 reflex action, or consider that they are carried out by the muscle 

 itself and that the cord intervenes only so far as to increase, 

 maintain or diminish the irritability of the muscular substance, it 

 remains good that they are prominent whenever the conditions 

 increase the reflex or other excitability of the cord, and diminish 

 or disappear when the conditions lower or abolish that excita- 

 bility. 



599. Disease in man reveals other actions of the spinal cord 

 which bear features different from those of an ordinary reflex 

 movement, and yet have been described as reflex in nature. For 

 instance certain affections of the cord are characterized by the 

 legs becoming rigid in extreme extension, the rigidity of the 

 straightened limbs being often so great, that when a bystander 

 lifts up one leg from the bed, the other leg is raised at the same 

 time. The rigidity is due to the extensor muscles being thrown 

 into a state of contraction, which is so uniform and long con- 

 tinued that it may be spoken of as a " tonic " contraction ; such 

 .a tonic rigidity may however be replaced by a series of rhythmic, 

 " clonic," contractions. It has sometimes been observed that the 

 limbs when flexed are supple and free from rigidity, but that 

 rigidity sets in so soon as they are brought into the position of 

 extension, the leg becoming suddenly fixed and straight somewhat 

 in the way that a clasp-knife springs back when opened. It 

 seems clear that the peculiar contraction is carried out by 

 means of the spinal cord, but the whole action, though it is 

 often spoken of as a 'muscle-reflex,' is very unlike an ordinary 

 reflex movement. In an ordinary movement an extensor is 

 brought into action when a limb is flexed, not when it is already 

 extended ; and if in a reflex act the condition of the muscle 

 about to be thrown into action determines in any way the 

 discharge of impulses from the reflex centre, we should expect 

 that the stretching of an extensor muscle by flexion, not its 

 relaxation by extension, would determine the discharge of 

 extensor impulses. In the case of the diseases in question just 

 the opposite seems to take place ; the position which appears to 

 determine the development of the remarkable contraction is 

 precisely that in which the strain upon the extensors is at its 

 minimum. It may be doubted, therefore, whether the word 



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