FUNCTIONS OF THE SPINAL CORD 917 



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 .ny power of originating movements or other actions 

 of this high degree of automatism. 



Muscular Tone or Tonus Postural Reflexes. It is generally 

 stated that 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. 

 This tonus has been clearly demonstrated for some muscles, though 

 not for all. 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, owing to a slight relaxation of the flexors. 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 of the limb be cut, its tone is completely lost, although 

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

 muscles in this classical experiment 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 subject is 

 considered here and not with the ordinary reflexes, partly to em- 

 phasize by a notable example the gradual abridgement of the sup- 

 posed automatic functions of the cord, which has been already 

 referred to, but partly because these reflexes have certain peculiari- 

 ties which distinguish them from the reflexes manifested by move- 

 ments. It has since been shown that this reflex tonus of skeletal 

 muscle is a postural reflex i.e., a reflex contraction of muscles asso- 

 ciated with the maintenance of the posture of a part or of the animal 

 as a whole. It can be readily demonstrated in the decerebrate 

 mammal that the tonus of the extensor muscles of the limbs is a 

 reflex sustained through the afferent fibres of the muscles themselves, 

 including those from the tendons and the neighbouring joints (the 

 proprioceptive system) and not through the cutaneous nerves. It 



