312 INNERVATION. [CHAP. XI. 



the anterior extremities produced, but the animal evinces unequi- 

 vocal signs of pain: but, when the posterior segment is irritated, 

 the animal seems not only insensible to pain, but unconscious even 

 of the movements that have been excited in the posterior extre- 

 mities. 



Nothing can be more conclusive than such an experiment, in 

 illustration of the fact that connexion with the encephalon is neces- 

 sary to sensation : and that movements, not only without volition, 

 but even without consciousness, may be excited by stimulating the 

 posterior segments. 



Direct irritation of the spinal cord is capable of exciting these 

 movements as much as when the stimulus is applied to the skin. 



When the spinal cord is removed, all these motions cease ; no 

 movement of any kind, voluntary or involuntary, can then be ex- 

 cited, except by directly stimulating the muscles, or the motor 

 nerves by which the muscles are supplied. Division of all the roots 

 of the nerves at their emergence from the cord produces precisely 

 the same effect. Under such circumstances no motion can be ex- 

 cited by stimulation of the surface, nor by stimulation of the cord 

 itself; and this fact may be regarded as an unequivocal proof that 

 the nerves, in ordinary actions, are propagators of the change pro- 

 duced by impressions to or from the centres; and that in the 

 physical nervous actions the stimulus acts not from one nerve to 

 another directly, but through the afferent nerve upon the centre, 

 by which the motor nerve is excited. 



From these details we may draw the following conclusions : 1, 

 that the spinal cord, (we use the term in its simple anatomical 

 sense, the intra-spinal nervous mass), in union with the brain, is 

 the instrument of sensation and voluntary motion to the trunk and 

 extremities ; 2, that the spinal cord may be the medium for the 

 excitation of movements, independently of volition or sensation, 

 either by direct irritation of its substance, or by the influence of a 

 stimulus conveyed to it from some surface of the trunk or ex- 

 tremities by its nerves distributed upon that surface. 



This latter office of the cord, although recognized by Whytt, 

 Prochaska, Blane, and Flourens, had not attracted all the notice 

 which its great importance merits, until the reseaches of Dr. Mar- 

 shall Hall and Professor Mliller drew attention to them ; and to 

 these physiologists, but especially to the former, much praise is due 

 for the zealous and efficient manner in which they have investigated 

 the subject. 



The class of actions which take place in virtue of this power of 



