402 THE NERVOUS SYSTEM. 



may be repeated a great number of times, until the irritability of the 

 nervous system is exhausted, or until some structural change has taken 

 place in the tissues. 



In the movements thus produced after decapitation there are two 

 important peculiarities : 



First, they are never spontaneous ; but are excited only by the appli- 

 cation of an external stimulus. The decapitated frog, if left to itself, 

 remains motionless, in a nearly natural attitude, without any tendency 

 to alter its position. Each application of stimulus causes a movement, 

 after which the limbs resume their condition of quiescence, until a repe- 

 tition of the stimulus calls out a new movement. 



Secondly, the action is not produced by direct excitement of the mus- 

 cles. The stimulus is applied to the integument of the foot, and the 

 muscles of the leg and thigh contract in consequence. This shows 

 that both sensitive and motor nerves take part in the process. The 

 sensitive fibres of the integument receive the impression and convey it 

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

 muscles in a different part. Even 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 link in the chain of communication. 

 For if the posterior limb be left uninjured, while its connection with 

 the cord is severed by dividing the sciatic nerve in 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 be destroyed by a stilet introduced into 

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

 of the integument will no longer produce muscular reaction in the limb. 

 The muscles can then be excited only by a stimulus applied to them- 

 selves, or to their motor nerves. 



These facts show that the phenomena in question are due to a reflex 

 action, in which three different nervous elements 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 between the 

 two, in which the reflex action is accomplished. The nervous centre, 

 in this instance, is the gray substance of the spinal cord. 



It is evident, accordingly, that consciousness is not necessary for the 

 reception of sensitive impressions by a nervous centre ; and also that 

 motor impulses may originate in a nervous centre without volition. 

 The reflex action of the spinal cord is both unconscious and involun- 

 tary ; and yet it is completely efficient, and produces muscular action 

 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 temporary depression or suspension by shock or injury to the 



