338 INNERVATION. [CHAP. XI. 



vesicular matter of the ganglia necessary for the movements of the 

 wasp in pursuit of its prey, had already been excited by a power- 

 ful stimulus of volition, which continued even after the removal of 

 the centre from which it had emanated. 



So similar is the change which a physical stimulus can excite in 

 the gray matter to that produced by the influence of the will, that, 

 as has been often remarked, the actions excited in decapitated 

 animals present a striking resemblance to the ordinary voluntary 

 movements. When a certain portion of the skin is irritated, the 

 animal pushes against the offending substance, as if trying to 

 remove or displace it. If the anus be irritated, both legs are ex- 

 cited to action. It may also be observed, that the same motions 

 follow the same irritations of the skin. If, in a frog, the seat of 

 irritation be on the right side, the corresponding hind-foot will be 

 raised, as if to remove the irritating cause. The exact resemblance 

 of these to voluntary movements seems to admit of being explained 

 only on the supposition that the same fibres are employed in the 

 execution of both. 



It must be borne in mind, that, while this hypothesis rejects the 

 class of sensori- volitional fibres which pass with the spinal nerves along 

 the cord into the brain, it admits the existence of only three orders 

 of fibres implanted in the various segments of the cord, viz. those 

 at once sensitive and excitor ; those at once for voluntary and in- 

 voluntary motion; and commissural fibres. Moreover, it is not 

 intended by this hypothesis to assume that the intervention of 

 sensation (i.e. the perception of an impression by the mind) is neces- 

 sary for the production of those muscular actions which are excited 

 by stimulation of the surface. No more is affirmed than that the 

 same stimulus to the sensitive nerve which can and does excite a 

 sensation, may simultaneously, but independently, cause a change in 

 the vesicular matter which shall stimulate the motor nerves ; and 

 that this change is of the same kind as that which the will may 

 excite, and affects the same motor nerves. 



Lastly, this hypothesis involves the enunciation of a highly im- 

 portant proposition with reference to nervous centres. It is this : 

 that all the centres which are connected to the brain by commissural 

 fibres, are thereby submitted to, and brought into connexion with, 

 the mind, to an extent proportionate to the number of connecting 

 fibres, so that voluntary impulses act upon them as part and parcel 

 of the centre of volition ; and sensitive impressions, in affecting 

 them, affect the sensorium commune simultaneously. 



In voluntary actions, then, it may be stated, that while the 



