THE SPINAL CORD. 407 



the body strikes the ground, giving a fresh stimulus for extension of 

 the limbs. But in these animals, the muscular actions required for the 

 attitude and locomotion are very simple. In warm-blooded quadrupeds 

 and in man, on the other hand, they are more complex, and volition is 

 essential for either standing or progression. Both these powers are 

 consequently abolished by decapitation. 



But, although the voluntary impulse is necessary for the acts of stand- 

 ing or walking, it is not concerned in the details of their mechanism. 

 Once excited, the nervous action by which walking is accomplished 

 may be kept up without mental effort or attention. All we have to 

 do is to commence the process by an act of volition, and the requisite 

 nervous machinery is set in motion. If we decide to turn a corner, all 

 the muscular combinations necessary for that purpose are effected with- 

 out the intermediate intervention of the will. This secondary action, 

 by which motor impulses are combined in the movement of the limbs 

 and trunk, is dependent on the action of the spinal cord. 



The precise mode in which this is accomplished is not positively 

 known. The most probable explanation is that it is due to a constant 

 reflex activity of the cord, by which the muscles of the body and limbs 

 are maintained in the proper degree of tension or relaxation ; and that 

 different parts of the cord are united with each other for this purpose 

 by longitudinal fibres in the posterior columns. 



According to this view, the fibres in question run a comparatively 

 short course in the posterior columns, each one, after leaving the gray 

 substance at one point, again entering it a few centimetres higher up ; 

 but, as they follow each other in continuous series, they form a mass 

 of connecting strands throughout the cord. It is certain that at the 

 borders of the gray substance and white columns of the cord there 

 are fibres passing obliquely from one to the other ; and this is espe- 

 cially true of the posterior columns and posterior horns. It is not pos- 

 sible, by any means of microscopic investigation now in use, to see the 

 origin and termination of these fibres ; but their existence is rendered 

 probable by several well-established experimental and pathological facts. 



I. The posterior columns of the cord, as shown by experiment, are 

 not the channels for either sensibility or voluntary motion. But, 

 according to Yulpian,* although a section of these columns at any one 

 point produces no paralysis, in the ordinary sense, if they be divided 

 by several transverse sections, two or three centimetres apart, there 

 is a remarkable disturbance in the power of locomotion, like that 

 which would be due to a want of muscular harmony. 



II. Destructive lesions situated at any point in the spinal cord give 

 rise to secondary degenerations like those already described (page 394), 

 which are " ascending" or " descending" in various parts of its lon- 

 gitudinal columns. According to Charcot,f such secondary degenera- 



* Lefons sur la Physiologie du Systeme Nerveux. Paris, 1866, p. 381. 

 f Leons sur les Localisations dans les Maladies du Cerveau et de la Moelle 

 Epiniere. Deuxieme Partie. Paris, 1880, p. 243. 



