REFLEX FUNCTION OF THE SPINAL CORD. 501 



direct from the Encephalon to the nerve-roots, their proportion must be 

 very small. And we shall see that all the phenomena which have been 

 supposed to indicate an immediate communication between the brain 

 and the nerves, admit of another and more satisfactory explanation^ 



883. It was supposed by Sir C. Bell (who was the first to determine 

 the relative functions of the two roots of the spinal nerves in Verte- 

 brated animals), that the anterior columns of the Spinal cord have a 

 function corresponding to that of the anterior roots of the spinal nerves ; 

 and the posterior columns with the posterior roots. But from the diffi- 

 culty of tracing the connexion between the longitudinal fibres of the 

 cord and any portion of the roots, it is at present impossible to say 

 how far there is any anatomical reason for the assumption of this cor- 

 respondence ; and it is quite certain that the physiological facts at 

 present known, from observation of the effects of disease or injury upon 

 different tracts of the spinal cord, do not bear out the supposition. As 

 to what the precise functions of the several columns are, however, it is 

 not easy to form any other conjecture, that shall be consistent with all 

 the phenomena at present known. 



884. Of the particular Reflex actions to which the Spinal Cord (using 

 that term in its limited sense, as excluding the Medulla Oblongata) is 

 subservient, those most connected with the organic functions have 

 already been noticed. They are chiefly of an expulsive kind ; being 

 destined to force out the contents of various cavities of the body. Thus 

 the ordinary acts of defecation and urination, the ejaculatio seminis and 

 parturition, are all reflex actions, over which the will has a greater or 

 less degree of control ; being able to keep the two former ones in check, 

 so long as the stimulus is not very violent, and being also capable of 

 effecting them by itself; but having no control over the two latter, 

 either by way of acceleration or prevention, when once the stimulus by 

 which they are excited has come into full action. The movements of 

 the posterior extremities are among the most remarkable of those, which 

 seem due to the action of the proper Spinal Cord. It has been already 

 noticed, that these may be excited, even in Man, when the spinal 

 cord has been severed in the middle without injury to its lower segment ; 

 and it is remarkable, that gentle stimuli, applied to the skin of the sole 

 of the foot, appear the most capable of producing them. We have seen 

 how completely, in the lower animals, the acts of progression may be 

 sustained, by the repeated stimulus of the contact of the ground, or of 

 fluid, without any influence from the cephalic ganglia ; the power of 

 these being limited, it would seem, to the control and direction of them. 

 And there is strong reason to believe that so far as the ordinary acts 

 of locomotion are concerned, the movements of the inferior extremities 

 in Man may be performed on the same plan, being continued by the 

 reflex influence of the successive impulses of the feet upon the ground, 

 when once set in action by the will, whilst we are walking steadily on- 

 wards, the mind being at the same time occupied by some train of 

 thought, which engrosses its whole attention. -There are few persons, 

 to whom it has not occasionally happened that, on awaking (as it were) 

 from their revery, they have found themselves in a place very different 

 from that to which they had intended going ; and even when the con- 



