460 THE SPINAL CORD. 



inward ; after which the motor fibres transmit an outward stimulus to 

 the muscles in a different part of the limb. Even the other limbs, as 

 already mentioned, may be set in motion by an irritation applied to 

 the integument of one. 



Furthermore, the nervous action is not transmitted, in these cases, 

 directly from the integument to the muscles; it passes through the 

 spinal cord, which thus forms a necessary link in the chain of communi- 

 cation ; for if the posterior limb be left uninjured, while its connection 

 with the cord is severed by dividing the sciatic nerve in the cavity of 

 the abdomen, no further action can be excited, and the limb remains 

 motionless whatever irritation be applied to the integument. 



Lastly, if the spinal cord itself be destroyed by the introduction of a 

 stilet into the spinal canal, this also puts an end to the phenomena, and 

 irritation of the integument will no longer produce a muscular reaction. 

 After that, the muscles can only be excited to contraction by a stimulus 

 applied directly to themselves, or to their motor nerves. 



All these facts show that the phenomena in question are due to the 

 reflex action of a nervous centre, in which three different nervous ele- 

 ments take part ; namely, first, the sensitive nerve fibres, conveying an 

 impression inward from the integument ; secondly, motor nerve fibres, 

 transmitting a stimulus outward to the muscles ; and, thirdly, a nervous 

 centre which intervenes between the two, and in which the reflex action 

 is accomplished. The nervous centre, in this instance, is the gray sub- 

 stance of the spinal cord. 



It is evident, accordingly, that consciousness is not a necessary ac- 

 companiment to the reception of sensitive impressions by a nervous 

 centre ; and that a motor impulse may also originate in a nervous centre 

 without the act of volition. The reflex action of the spinal cord takes 

 place without either consciousness or volition ; and yet it is completely 

 efficient, and produces muscular contraction at once on the application 

 of a stimulus to the skin. 



Diminution or Increase of Beflex Action in the Cord. The reflex 

 action of the spinal cord, like other forms of nervous activity, may suffer 

 a temporary depression, or even total suspension, by any shock or injury 

 to the system at large. The operation of separating the head from the 

 trunk in the frog will often be followed, for a few moments, by an 

 interval of complete nervous paralysis, in which no phenomena of reac- 

 tion can be obtained. Even injuries in which the nervous centres are 

 not directly interested, such as the opening of the abdomen and the 

 removal of the abdominal organs, may produce a similar temporary 

 effect. In some instances the duration of this period of depression is 

 very short, so as to be almost imperceptible ; in others it lasts for 

 several minutes. After it has passed off, the reflex irritability of the 

 cord returns to its natural condition, and, if the cord itself have been 

 wounded or divided, may even be perceptibly increased in intensity. 



It is for this reason that the reflex action of the cord often seems to 

 be more vigorous and prompt in the frog after the removal of the head, 



