THE CENTRAL NERVOUS SYSTEM 713 



the domain of purely automatic action has been narrowed, 

 that of reflex action has extended, until the controversy as 

 to the boundaries between the two seems not unlikely to be 

 ended by the absorption of the automatic in the reflex. 

 And as we seem almost driven to conclude that from the 

 anatomical standpoint the nervous system is essentially a 

 vast collection of looped conducting paths, each with an 

 afferent portion, an efferent portion, and connections between 

 them formed by the end arborizations of the axons and 

 their collaterals, the dendrites and the cell-bodies, so it may 

 be that no true physiological automatism really exists either 

 in cord or brain, that every form of physiological activity 

 muscular movement, secretion, intellectual labour, conscious- 

 ness itself would cease if all afferent impulses were cut off 

 from the nervous centres. But there are certain groups of 

 actions so widely separated from the most typical reflex 

 actions that, provisionally at least, they may be distin- 

 guished 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 

 completely relaxed ; its fibres are in a condition of slight 

 tonic contraction, 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 continual 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 com- 

 pletely 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 



