402 THE FUNCTIONS OF THE ROOTS OF SPINAL NERVES. 



will be found that the irritability of the nerve in this region 

 has been very considerably lowered ; or, if the polarizing cur- 

 rent be strong enough, and the pair of polarizing electrodes 

 far enough apart, has been suspended altogether. Contractions 

 in the muscle are either entirely absent when a shock is sent 

 through B, or only appear when the shock is very strong. At 

 the same time it will be found that the minimum stimulus of 

 A is not very different from what it was before. A rather 

 stronger stimulus is required to produce a contraction, but the 

 difference is strikingly less than that in the case of the elec- 

 trodes B, and even this difference may be accounted for by 

 considering that the electrodes A stimulate both the muscular 

 fibres and the intra-muscular nerve fibres, and that the com- 

 bined effect is therefore greater when the intra-muscular nerves 

 are intact than when they are paralyzed by the ascending cur- 

 rent. 



Thus the ascending current will, if strong enough, suspend 

 the irritability of the nerve fibres supplying a muscle, and yet 

 will leave the muscle but little altered in its susceptibility to 

 direct stimulation. This again is an argument in favor of 

 " independent muscular irritability." 



The same view is supported by the facts that the chemical 

 irritants of nerve and muscle are not identical (see Chapte 

 .XXX.. uftts. V.-V1I.) ; that the lower part of the sartori 

 of young frogs in which no nerve fibres can be detected, is 

 susceptible of chemical stimulation ; and that the idio-muscu- 

 lar contraction ma}- be called forth in muscles the nerves of 

 which have completely lost their irritability. (Chapter XXX., 

 Obs. IV.) 



CHAPTER XXXII. 

 THE FUNCTIONS OF THE ROOTS OF SPINAL NERVES. 



THE posterior root of a spinal nerve is said to be sensory, 

 i. t\. to serve as the path along which alone centripetal influ- 

 ences pass on their way from the peripheral nerve terminations 

 to those central organs, in which they become transformed 

 into sensations, or give rise to reflex actions, etc. The anterior 

 root is said to be motor, i. e., to serve as the path along which 

 :lonc ct'ittrifiHjal impulses pass, on their way from the central 

 organs to the nerve terminations in muscles, etc. The truth 

 of this absolute distinction in function between the two roots 

 may readily be shown in the frog. 



The results are most clear and distinct when the organs of 



