SENSORY GANGLIA IN VOLUNTARY MOVEMENT. G69 



to the movements and distracts the mind from the sense of its danger. The 

 degree in which the "muscular sense" is alone sufficient for the guidance of 

 such movements, when the, mind has no consciousness of the danger, and 

 when the visual sense neither affords aid nor contributes to distract the at- 

 trntion, is remarkably illustrated by the phenomena of Somnambulism ; for 

 the sleep-walker traverses, without the least, hesitation, the narrow parapet 

 of a house, .and crosses narrow and insecure planks, clambers roofs, etc., 

 under circumstances that clearly indicate the nature of the guidance by 

 which he is directed. The dependence of our ordinary power of maintaining 

 our equilibrium, upon the combination of the guiding sensations derived 

 through the sight and the touch, is further well illustrated, as Mr. Mayo has 

 pointed out (loc. cit.), by what happens to a landsman on first going to sea. 

 " It is long before the passenger acquires his 'sea legs.' At first, as the ship 

 moves, he can hardly keep his feet ; the shifting lines of the vessel and sur- 

 face of the water unsettle his visual stability; the different inclinations of 

 the planks he stands on, his muscular sense. In a short time he learns to 

 disregard the shifting images and changing motions, or acquires facility in 

 adapting himself (like one on horseback) to the different alterations in the 

 line of direction in his frame." And when a person who has thus learned 

 by habit to maintain his equilibrium on a shifting surface, first treads upon 

 firm ground, he feels himself almost as much at fault as he did when he first 

 went to sea, and it is only after being some time on shore, that he is able to 

 resume his original manner of walking. Indeed, most of those who spend 

 the greater part of their time at sea, acquire a peculiar gait, which becomes 

 so habitual to them that they are never able to throw it off. 



ooO. But, further, there is very strong physiological evidence that the 

 Sensory Ganglia are not merely the instruments whereby our voluntary 

 movements are directed and controlled, in virtue of the guiding sensations 

 which they furnish, but that they are actually the immediate centres of the 

 motor influence which excites muscular contractions, in obedience to impulses 

 transmitted downwards from the Cerebrum. It has usually been considered 

 that the Cerebrum acts directly upon the muscles, in virtue of a direct con- 

 tinuity of nerve-fibres from the gray matter of its convolutions, through the 

 Corpora Striata, the motor tract of the Medulla Oblongata, the anterior 

 portion of the Spinal Cord, and the anterior roots of the nerves ; and that 

 in the performance of any voluntary movement, the Will determines the 

 motor force to the muscle or set of muscles by whose instrumentality it may 

 be produced. To this doctrine, however, the anatomical facts already stated 

 constitute a very serious objection ; for the motor tract cannot be stated with 

 certainty to have any higher origin than the Corpora Striata; and it is 

 impossible to imagine that the fibres which converge towards the surface of 

 these bodies from all parts of the Cerebrum, can be so closely compacted 

 together as to be included in the motor columns of the Spinal Axis. The 

 fact would rather seem to be, that these converging fibres bear the same 

 kind of anatomical relation to the Corpora Striata and the other Sensorial 

 centres of motor power, as do the fibres of the afferent nerves which proceed 

 to them from the retina, the Schueiderian membrane, and other periphera-l 

 expansions of nervous matter ; and hence we might infer that the nerve-force 

 generated in the 'con volutions, instead of acting immediately on the motor 

 nerves, is first directed towards the Automatic centres, and excites the 

 same kind of motor response in them, as would be given to an impression 

 transmitted to them through a sensory nerve. We shall find that such a 

 view of the structural arrangements of these parts is in remarkable accord- 

 ance with their functional relations, as indicated by careful analysis of the 

 mechanism of what is commonly regarded as "voluntary" movement. The 



