THE CENTRAL NERVOUS SYSTEM 813 



isolated from other neurons. The more the nervous system is 

 investigated, the deeper grows the conviction of its essential 

 solidarity, the more clearly it displays itself as a single mechanism, 

 the most distant parts of which are intricately knit together. 

 But there are certain groups of actions so widely separated from 

 the most typical reflex actions that, provisionally at least, they 

 may be distinguished as automatic. Such are the voluntary 

 movements, and certain involuntary movements, like the beat 

 of the heart. And we may proceed to inquire whether the spinal 

 cord has any power of originating movements or other actions of 

 this high degree of automatism. 



Muscular Tone. So long as a muscle is connected with 

 the spinal segment from which its nerves arise, it is never com- 

 pletely relaxed ; its fibres are in a condition of slight tonic con- 

 traction, and retract when cut. If a frog whose brain has 

 been destroyed is suspended so that the legs hang down, and one 

 sciatic nerve is cut, the corresponding limb may be observed 

 to elongate a little as compared with the other. At one time 

 this tone of the muscles was supposed to be due to the con- 

 tinual automatic discharge of feeble impulses from the grey 

 matter of the cord along the motor nerves. But it has been 

 proved that if the posterior roots be cut, or the skin removed 

 from the leg, its tone is completely lost, although the anterior 

 roots are intact. So that the tone of the skeletal muscles 

 depends on the passage of afferent impulses to the cord, and 

 must be removed from the group of automatic actions and 

 included in the reflexes. 



The ' rigidity ' of the muscles, often observed in paralysis 

 from lesions of the central system, and denominated ' early ' 

 or ' late ' according as it comes on within a few days or a few 

 weeks after the occurrence of the lesion, is also probably in part, 

 at least, a reflex phenomenon, although, perhaps, possessing 

 some of the characters of a tonic contraction due to automatic 

 discharge from the spinal centres. For in such cases ' myotatic 

 irritability ' is increased ; the knee-jerk is exaggerated ; a finger- 

 jerk may be elicited by tapping the wrist, an arm -jerk by striking 

 the skin over the insertion of the biceps or triceps, ankle-clonus 

 by flexing the foot (Gowers) . Now, myotatic irritability depends 

 on reflex muscular tone (p. 802). 



It is probable that the tone of such visceral muscles as the 

 sphincters of the anus and bladder has also a reflex element, 

 and possible that the same is true of the tone of the smooth 

 muscular fibres of the bloodvessels on which the maintenance 

 of the mean blood- pressure so largely depends (p. 169). 



Trophic Tone. The degenerative changes that occur in 

 muscles, nerves, and other tissues when their connection with 



