320 INNERVATION. [CHAP. XI. 



must be through their constituent fibres that the cerebellum exerts 

 its influence on the nerves of the lower extremities, and of other 

 parts concerned in the locomotive function. 



The nearly uniform size of the posterior columns in the different 

 regions of the cord has been already remarked as unfavourable to 

 their being channels of sensation. But this anatomical fact may be 

 adduced as a good argument in support of the hypothesis which we 

 are now discussing. It is 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. 



In examining a transverse section of the lumbar region of the 

 cord we observe a great predominance of its central gray matter ; 

 the posterior columns appear large, and the antero-lateral columns 

 inadequate in proportion to the large roots of nerves which emerge 

 from it. Now, an analysis of the locomotive actions renders it 

 highly probable, that they are partly of a volitional character and 

 partly dependent on the inherent power of that segment of the cord 

 from which the lower extremities derive ther nerves. In progression 

 there are two objects to be attained, to support the centre of 

 gravity of the body, and to propel it onward; the former object 

 requiring, first, that the muscles of the lower extremities, the pillars 

 of support to the trunk, should be well contracted, in a degree 

 proportionate to the weight they have to sustain. Those actions 

 by which the trunk is balanced upon the limbs, and by which the 

 movements of progression are effected, are subsequently called into 

 play through mental influence. The contraction of the muscles of 

 the limbs seems well provided for in an arrangement for the de- 

 velopment of nervous power by a stimulus propagated to the centre. 

 This 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 downwards. It is well known that physical nervous 

 actions are more developed in the lower than 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 assigned for 

 this peculiarity, unless it have reference to the locomotive actions ; 

 and the great development of the vesicular matter in this region 

 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 play in balancing the trunk, and in harmonizing its movements 

 with those of the lower extremities. 



