FUNCTIONS OF THE SPINAL CORD 889 



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

 that no strict physiological automatism really exists either in cord 

 or brain, that every form of physiological activity muscular move- 

 ment, secretion, intellectual labour, consciousness itself would 

 cease if all afferent impulses were cut off from the nervous centres. 

 Assuredly no neuron is entirely isolated from other neurons. The 

 more the nervous system is investigated, the deeper grows the con- 

 viction 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 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 corre- 

 sponding 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 re- 

 moved 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. 



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. 183). 



Trophic Tone. The degenerative changes that occur in muscles, 

 nerves, and other tissues when their connection with the central 

 nervous system is interrupted have been already referred to (p. 777). 

 It is possible to explain these changes in some cases without the 

 assumption that tonic impulses are constantly passing out from the 

 brain and cord to control the nutrition of the peripheral organs; 

 and we have seen that there is no real evidence of the existence of 

 specific trophic fibres. But the degeneration of muscles after sec- 

 tion of their motor nerves is difficult to understand except on the 

 hypothesis that impulses from the cells of the anterior horn influence 

 their nutrition. The only question is whether these are the im- 



