ACTION AS A NEEVOUS CENTRE. 459 



Action of the Spinal Cord as a Nervous Centre. 



So far as the spinal cord is concerned in the phenomena of sensation 

 and voluntary motion, it acts as a medium of communication between 

 the brain, where consciousness and volition reside, and the integument 

 and muscles of the external parts. Its complete division accordingly at 

 any point destroys this communication, and suspends the nervous func- 

 tions dependent upon it ; so that the commands of the will are no longer 

 transmitted to the muscles below, and the individual is incapable of 

 perceiving impressions made upon the integument of the paralyzed 

 parts. But after such an operation motion is not altogether abolished 

 in the body and limbs ; and impressions conveyed by the sensitive 

 nerve fibres, though no longer perceived by the individual, are still 

 capable of producing an effect, and of exciting a reaction in the organs 

 of movement. These phenomena, which take place without the inter- 

 vention of the brain, are produced by the action of the spinal cord as a 

 nervous centre, and are due to the independent properties of its gray 

 matter. 



Eeflex Action of the Spinal Cord. If a frog be decapitated, and 

 allowed to remain at rest for a few moments, until the depressing effects 

 of the shock upon the nervous system have passed off, movements can 

 readily be excited in either the anterior or posterior limbs. If the skin 

 of one of the feet be irritated by pinching with a pair of forceps, or by 

 immersing it in a weak acidulated solution, the leg is immediately 

 drawn upward toward the body, as if to escape the source of irritation. 

 If the stimulus applied be of slight intensity, the corresponding leg only 

 will move ; but if it be more severe in character, motion will often be 

 produced in the corresponding limb on the opposite side, or even in all 

 the extremities at once. These phenomena may be repeated a great 

 number of times, until the irritability of the nervous system has been 

 exhausted, or until some structural change has taken place in the 

 tissues. 



Two important peculiarities are noticeable in the movements thus 

 produced after decapitation : 



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

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

 always remains perfectly motionless, in a nearly natural attitude, but 

 without any tendency to alter its position. Each application of a 

 stimulus causes a movement, after which the limbs again assume a con- 

 dition of quiescence until a repetition of the stimulus calls out a new 

 movement. 



Secondly, the muscular action thus manifested is not produced by a 

 stimulus directly applied to the muscles themselves. The stimulus is 

 applied to the integument of the foot, and the muscles of the leg and 

 thigh are contracted in consequence. This shows that both sensitive 

 and motor nerve fibres take part in the action. The sensitive fibres 

 distributed to the integument first receive the impression and convey it 





