CHAP. XI.] FUNCTIONS OF THE CORD. 311 



tion of instances, and in the most marked way. In most of the 

 cases of hemiplegia the surface retains its sensibility; but in most 

 of those of paraplegia sensibility is much diminished or completely 

 destroyed. 



In the anencephalic foetus, in which all the encephalon but part 

 of the medulla oblongata is wanting by congenital defect, actions 

 take place in obedience to stimuli propagated to the cord from some 

 surface, or applied directly to it; but no movements are seen which 

 can be supposed to originate in an effort of the will, nor is there 

 any proof of the existence of sensibility. 



Other facts may be adduced in evidence of the involuntary na- 

 ture of these movements. 



It is remarkable that actions of this kind will continue to be 

 manifested after decapitation, not only in the trunk, but also in 

 segments of it with which a portion of the spinal cord remains in 

 connexion. If the body of a snake or an eel be divided into 

 several segments, each one will exhibit movements for some time, 

 upon the application of a stimulus. The same thing may be 

 observed in frogs, salamanders, turtles, and other cold-blooded 

 creatures. In birds and mammalia, however, they are less con- 

 ^^picuous, because in them the nervous power is so soon extinct. 



These facts suggest an obvious comparison between the spinal 

 cord of vertebrate animals and the abdominal ganglionic chain of 

 articulate irivertebrata. In the latter, each segment of the body has 

 its proper ganglionic centres, and is, therefore, to a certain extent 

 independent of the rest. Every schoolboy has witnessed the wri things 

 of an earthworm, which his mischievous propensity has prompted 

 him to divide into several pieces. Movements will continue in each 

 piece so long as the irritation produced by the subdivision re- 

 mains ; and, after that has ceased, movements may be excited in 

 any segment by stimulating its surface. These movements seem 

 precisely analogous to those which may be excited in the sub- 

 divisions of the trunk of a vertebrate animal. The spinal cord, 

 then, may be viewed as one continuous centre, made up of a number 

 of segments fused together at their extremities. In the articu- 

 late ganglionic chain the centres of the segments remain distinct, 

 although connected by fibres which pass from one to the other. 



When the spinal cord is divided about its middle, a remarkable 

 difference may be noticed in the effects of irritation on the anterior 

 and the posterior segment, as shewn in some of Flouren's experi- 

 ments. When the anterior segment (that which still retains its 

 connexion with the brain) is irritated, not only are movements of 



