CHAP, i.] THE SPINAL CORD. 717 



matter of words whether we speak of the maintenance of tone 

 as an automatic or as a reflex action of the cord, We may, 

 however, distinguish the part played by the afferent impulses 

 in assisting the cord to a condition in which it is capable of 

 maintaining tone from the part played by an afferent impulse 

 in causing a reflex action ; in the former the action of the affer- 

 ent impulses seems analogous to that of a supply of arterial 

 blood in maintaining an adequate irritability of the nervous 

 substance, in the latter the afferent impulses lead directly to a 

 discharge of energy. And it is convenient to distinguish the 

 two things by different names. 



471. The close connection between tone and reflex action 

 is shewn by the fact that some observers contend that the 

 4 knee-jerk ' and similar 4 tendon-phenomena ' are not instances 

 of reflex action. They maintain that the contraction of the 

 muscle is an example of the direct stimulation of the muscle 

 by the vibrations set up in the tense tendon when it is sharply 

 struck or suddenly pulled ; and they explain the dependence 

 of the act on the spinal cord by attributing variations in the 

 response of the muscle to variations in the tone of the muscle, 

 the tone being dependent on the- spinal cord. 



472. 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 charac- 

 terized 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 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 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 pecu- 

 liar contraction is carried out by means of the spinal cord, but 

 the whole action, though it is often spoken of as a 4 muscle- 

 reflex,' is very unlike an ordinary reflex movement. In an 

 ordinary movement an extensor is brought into action when a 

 lirnb 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 



