THE AUTOMATIC ACTIONS OF THE SPINAL CORD. 583 



of animals they disappear when the spinal cord is destroyed, or the nerves 

 going to the muscles are severed, or even when the posterior roots only are 

 divided. The measure of their development both in animals and in man is 

 also closely dependent on the condition of the spinal cord and of the central 

 nervous system generally. They may be increased or diminished, augmented 

 or inhibited by a coincident voluntary effort directed toward some other end, 

 or by the coincident development of a sufficiently distinct sensation. In 

 general it may be said that whatever favors 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 card (<?. </., locomotor ataxia) 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 irri- 

 tability of the muscular substance, it remains good that they are prominent 

 whenever the conditions increase the reflex or other excitability of the cord, 

 and dimmish or disappear when the conditions lower or abolish that excita- 

 bility. 



512. 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 by- 

 stander 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 continued 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 ex- 

 tension, the leg becoming suddenly fixed and straight somewhat in the way 

 that a clasp-knife springs back when opened It seems clear that the pecu- 

 liar 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 ex- 

 tended ; 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 deter- 

 mine 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 reflex should be used to denote such phenom- 

 ena ; but the phenomena themselves deserve attention, especially, perhaps, 

 as showing how in the disorders of the gray matter of the cord due to dis- 

 ease impulses or influences which are latent only in health become actual 

 and effective. 



It remains for us to speak of the part played by the spinal cord, as the 

 instrument of the brain, in the execution of voluntary movements and in the 

 development of conscious sensations ; but it will be best to consider these 

 matters in connection with the brain itself, to the study of which we must 

 now turn. 



