4-48 THE SPINAL CORD. 



before. All these facts prove that the path by which impulses for vol- 

 untary motion pass, from the brain and spinal cord to the muscles, is 

 exclusively the anterior root of the spinal nerve. 



On the other hand, if the posterior root be irritated, by pinching, 

 pricking, or galvanism, a sensation is produced, more or less acute ac- 

 cording to the amount and quality of the irritation applied. This sen- 

 sation, when of a certain intensity, is accompanied by movements. But 

 these movements are general and not necessarily confined to the part to 

 which the nerve is distributed; and if the corresponding anterior root 

 have been previously divided, this part will remain motionless, while 

 muscular contractions continue to be produced elsewhere. The move- 

 ments which follow irritation of the posterior root, accordingly, are 

 not produced directly by its influence, but are caused indirectly by 

 the reaction of the nervous centres. The only immediate result of irri- 

 tation of a posterior nerve root is a sensation, and this root is therefore 

 said to be " sensitive." 



Furthermore, if the posterior root be divided, the consequence is a 

 loss of the power of sensation in the corresponding region of the body. 

 Here also the effect is simply to cut off the medium of communica- 

 tion between the integument and the nervous centres ; since irritation 

 of that part of the divided nerve which is still attached to the spinal 

 cord produces a sensation as before. Thus the channel of communica- 

 tion for sensitive impressions, in this part of the nervous system, is 

 exclusively the posterior root of the spinal nerve. 



But just beyond the situation of the spinal ganglia, the two roots 

 unite to form the trunk of the spinal nerve. Here, the fibres of the 

 anterior and those of the posterior root become intimately mingled, and 

 inclosed within the neurilemma in such close juxtaposition that they 

 can no longer be separately irritated by artificial means. They pass, 

 still associated in this manner, into the branches and subdivisions of 

 the nerve ; and only finally separate from each other at its terminal 

 ramifications, where the sensitive fibres are distributed mainly to the 

 integument and the motor fibres to the muscles. 



The trunk and branches of a spinal nerve, therefore, outside the 

 spinal canal, contain both sensitive and motor fibres, and it is conse- 

 quently said to be a " mixed" nerve. It is both excitable and sensitive, 

 since its artificial irritation causes at the same time sensation and con- 

 vulsive movement ; and if it be divided, this injury is followed by the 

 loss of both sensibility and voluntary motion in the corresponding 

 parts. It is also important to remember that, in all these instances of 

 section of the trunk, or branches, or anterior or posterior roots of a 

 spinal nerve, the consequent loss of sensibility or motion is permanent 

 while the injury lasts ; and the nervous functions are not restored until 

 the divided nerve fibres have reunited, and have again acquired their 

 natural continuity of texture. This shows that the suspension of func- 

 tional activity is directly due to the division of the nerve fibres, and is 

 not produced by the sympathetic action of other parts. 



