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 commissuml 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 colurnns.
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 g,anglia, 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, vvhich 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 vvhich 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 deg,ree proportioned to the weight they have to sustain. The contrac tion of these muscles seems well provided for in an arrangement for the developement 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 ference to the locomotive actions, and the great development of the vesicular nervous matter in these regions betokens the frequent and energetic evolution of the nervous force. All the structural arrangements necessary for this purpose are found in the antero-lateral columns. The posterior columns come into exercise in balancing the trunk and in harmonizing its movements with those of the lower extre mities.