CHAP. XI.] MECHANISM OF A VOLUNTARY ACTION. 329 



voluntary motion do not pass beyond that particular segment of the 

 cord with which they are connected ; and that each segment of the 

 cord communicates readily with the brain through the horns of gray 

 matter, or through commissural fibres which pass between the seg- 

 ments of the cord, and from the upper segment of the latter to the 

 brain. The anatomy of the cord, so far as our present knowledge 

 extends, is favourable to this hypothesis, for it is much more pro- 

 bable that all the roots of the spinal nerves are implanted in their 

 proper segments of the cord, than that some pass up to the brain, 

 and others remain in the cord. The varying dimensions of the cord, 

 at different regions, disincline us to admit the existence of fibres 

 which are continued up into the brain from the spinal nerves. 

 It is impossible to understand the great superiority of size of the 

 lumbar portion over the dorsal segment of the cord, if we admit 

 that this latter segment contains, in addition to its own fibres 

 (sensori- volitional and excito-motory), the sensori- volitional fibres 

 of the lumbar swelling also. The fibres of sensation and volition, 

 which pass to the great lumbar and sacral nerves, could, in that 

 case, be only extremely few in proportion to the excito-motory ones ; 

 nor would they seem adequate to the motor and sensitive endow- 

 ments of the lower extremities ; where it must be admitted volition 

 and sensation enjoy an extensive sway. Moreover, it may be stated 

 that the great size of the lumbar swelling depends mainly on the 

 large quantity of vesicular matter which exists in it ; and the total 

 amount of fibrous matter is hardly so much as might be expected to 

 exist if the lower extremities and the pelvis were supplied with both 

 sensori-volitional and excito-motory fibres. 



It is very generally admitted, that the only channel by which the 

 will can influence the spinal cord is through the fibres of the ante- 

 rior pyramids, the greater number of which decussate each other 

 along the median line, as already explained in page 265. The 

 most frequent pathological phenomena favour this view. Now it 

 is in the highest degree improbable that these fibres, occupying so 

 si nail a space as they do, should form the aggregate of the voli- 

 tional fibres (still less of the sensori-volitional fibres) of the trunk 

 and extremities. It seems to us much more reasonable to regard 

 the fibres of the pyramids in the light of commissures, connecting 

 the gray matter of the cord with that of the brain, and serving 

 to associate these two great divisions of the cerebro-spinal centre 

 in the voluntary, if not in all the mental nervous actions. 



The mechanism of a voluntary action, in parts supplied with 

 spinal nerves, would be, according to this hypothesis, as follows : 



