100 PHYSIOLOGY. 



III. A set of excitor, or centripetal fibres, terminating in the true 

 spinal cord, or ganglion, and conveying impressions to it. 



IV. A motor, or centrifugal set, arising from the same ganglionic 

 centre (or true spinal marrow) and conveying the motor influence 

 reflected //•o?/^ it to the muscles. 



Of these, the first and third are united in the posterior, or affe- 

 rent root; the second ixndi fourth, in the aiiterior or effereyit roots. 



The functions of the I. and II. bundles have been treated of, when 

 speaking of the spinal cord, as a conveyor of nervous influence to 

 and from the brain. The III. and IV. are now to be considered, in 

 connexion with the cord, as an originator of nervous influence. 

 These latter, with the gray matter in the centre of the cord, consti- 

 tute the reflex syste?)i. 



The spinal cord has, in virtue of the gray matter in its composi- 

 tion, or the ganglionic cells collected in its interior, certain proper- 

 ties which characterize it as a central organ. It has a proper 

 inherent motory power, which it communicates to its nerves inde- 

 pendently of the brain, a fact which is proclaimed by the state of 

 permanent contraction of those muscles, the sphincter for example, 

 which depend most immediately on the spinal cord. If an animal 

 be stunned by a blow upon the head, or even decapitated in some 

 instances, it will still retain for some hours the power of moving the 

 -extremities, when the integument is pinched, but without the least 

 consciousness, or anything like volition ; the motions are automatic, 

 and proceed directly from the spinal cord, in consequence of an 

 excitement or stimulation of its substance effected through the fibres 

 of the spinal nerves that terminate in its gray substance ; the mo- 

 tions are pure reflex motions ; in other words, motions which arise 

 from stimuli conveyed to the spinal cord by centripetal or afferent 

 nerves, which stimuli are reflected from the cord by centrifugal or 

 efferent nerves. 



These movements will also continue if the spinal cord be cut 

 across, so as to make two segments, one for the upper and one for 

 the lower extremity ; each pair of members may be excited to move- 

 ment by stimuli applied directly to themselves. The same phenomena 

 are witnessed in the human subject when the spinal cord has suffered 

 injury, or disease in the middle of the back, provided the lower seg- 

 ment remain sound, and its nervous connexions with the limbs are 

 uninjured. These facts prove that sensation is not a necessary link 

 in the chain of reflex actions, all that is required being an afferent 

 fibre capable of receiving the impression, and conveying it to the 

 centre ; a ganglionic centre, composed of vesicular nervous sub- 

 stance into which the afl^erent fibre passes ; and an efferent fibre, 

 capable of transmitting the motor impulse from the ganglionic cen- 

 tre to the muscle which is to be thrown into contraction (Fig. 26). 



