PHYSIOLOGY OF THE NERVOUS SYSTEM. 



721Q 



When to these results, obtained from patho- 

 logical researches, we add those of experiment, 

 nothing is gained which can be favourable to 

 the attribute of sensitive power to the posterior 

 columns of the cord. Dr. Daly's experiments 

 on tortoises showed that movements might be 

 excited whether the anterior or the posterior 

 columns were irritated, much stronger mo- 

 tions being excited by the posterior than by 

 the anterior columns. Longet found that mo- 

 tions might be excited by irritation of the 

 posterior columns of the cord if the experiment 

 had been made immediately after the transverse 

 division of the cord, and he refers such mo- 

 tions, probably with justice, to an excited state 

 of the cord. After a little time this subsides, 

 and then M. Longet was able to pass the 

 galvanic current through each or both of the 

 posterior columns, without exciting any mo- 

 tions when the lower segment of the cord was 

 acted upon, but causing pain, as evinced by 

 loud cries and writhing of the body, when the 

 upper segment was tried. From experiments 

 of this kind no satisfactory deductions can be 

 made : to irritate the posterior columns of the 

 spinal cord in a living dog without affecting in 

 some degree the posterior roots of the nerves, 

 appears to me to be quite impossible, even in 

 the hands of the most practised vivisector. 



Neither anatomy, pathological observation, 

 nor experiment, lend sufficient countenance to 

 the doctrine of the identity of the function of 

 the posterior roots and posterior columns to jus- 

 tify us in concluding that these columns are the 

 ordinary channels for the transmission of the 

 sensitive impressions made upon the trunk and 

 extremities. 



I have long been strongly impressed with the 

 opinion that the office of the posterior columns 

 of the spinal cord is very different from any 

 yet assigned to them. They may be in part 

 commissural between the several segments of 

 the cord, serving to unite them and harmonize 

 them in their various actions, and in part sub- 

 servient to the function of the cerebellum in 

 regulating and co-ordinating the movements 

 necessary for perfect locomotion. 



This view is suggested by a comparison of 

 the spinal cord with the brain, and by the ana- 

 tomical connections of the posterior columns. 



The brain is an organ composed of various 

 segments, which are connected with each other 

 by longitudinal commissures. The cord is 

 obviously divisible into a number of ganglia, 

 each forming a centre of innervation to its 

 proper segment of the body. These portions 

 must be connected by similar longitudinal 

 commissures to those which confessedly exist 

 in the brain. If we admit such fibres to be 

 necessary to ensure harmony of action between 

 the several segments of the encephalon, there 

 are as good grounds for supposing their exist- 

 ence in the cord as special connecting fibres 

 between its various ganglia to secure consen- 

 taneousness of action between them. 



The attribute of locomotive power rests upon 

 the connection of the posterior columns with 

 the cerebellum, and the probable influence of 

 that organ over the function of locomotion and 



the maintenance of the various attitudes and 

 postures. If the cerebellum be the regulator of 

 these locomotive actions, it seems reasonable to 

 suppose that these columns, which are so largely 

 connected with it, each forming a large propor- 

 tion of the fibrous matter of each crus cerebelli, 

 should enjoy a similar function, and that, as they 

 are the principal medium through which the 

 cerebellum is brought into connection with the 

 cord, it must be through their constituent fibres 

 that the cerebellum exerts its influence on the 

 centre of innervation to the lower extremities 

 and other parts concerned in the locomotive 

 function, and on the nerves distributed to 

 these parts. 



The nearly uniform size of the posterior co- 

 lumns in the different regions of the cord, 

 whilst it may be noted as unfavourable to their 

 being viewed as channels of sensation, may be 

 adduced as a good argument in favour of their 

 being concerned in locomotion and acting as 

 commissural fibres. It is a fact worthy of 

 notice that these columns experience no marked 

 diminution in size until the large sacral nerves, 

 which furnish the principal nerves of the lower 

 extremities, begin to come off. The reason of 

 this is probably because the fibres of these co- 

 lumns connect themselves in great part with 

 the lumbar swelling of the cord, and some of 

 them perhaps pass into the sacral nerves. 



The following remarks will serve to explain 

 the manner in which the posterior columns may 

 contribute to the exercise of the locomotive 

 function. In examining a transverse section of 

 the cord in the lumbar region, we observe a 

 great predominance of its central grey matter; 

 the posterior columns appear large, and the 

 antero-lateral columns seem inadequate in pro- 

 portion to the large roots of nerves which 

 emerge from it. Now, an analysis of the loco- 

 motive actions shows, with great probability, 

 that they are partly of a voluntary character, 

 and partly dependent on the influence of phy- 

 sical impressions upon that segment of the 

 cord from which the nerves of the lower extre- 

 mities are derived. There are two objects to 

 be attained in progression, namely, to support 

 the centre of gravity of the body, and to propel 

 it onward. The former object is attained by 

 physical nervous actions, the latter by mental. 

 The support of the centre of gravity of the body 

 requires that the muscles of the lower extremi- 

 ties, the pillars of support to the trunk, should 

 be well contracted in a degree proportioned to 

 the weight they have to sustain. The contrac- 

 tion of these muscles seems well provided for 

 in an arrangement for the developemeut of 

 nervous power by a stimulus propagated to the 

 centre, and then reflected upon the motor 

 nerves of these muscles. The stimulus is 

 afforded by the application of the soles of the 

 feet to the ground ; it is therefore proportionate 

 to the weight which presses them down- 

 wards. It is well known that reflex actions 

 are more developed in the lower than in the 

 upper extremities, and the surface of the sole 

 of the foot is well adapted for the reception of 

 sensitive impressions. No object can be as- 

 signed for this peculiarity, unless it have re- 



