722H 



PHYSIOLOGY OF THE NERVOUS SYSTEM. 



the lumbar portion perfectly whole, the sphincter 

 will nevertheless be paralysed. In the former 

 instances, the centre of volition in the cranium 

 is diseased ; in the latter, the defect consists in 

 the destruction of the communication of the 

 brain with that portion of the cord in which 

 the nerves of the sphincter muscle are im- 

 planted. 



An examination of the action of the sphinc- 

 ter will show, as has been already noticed, that 

 the anus is kept closed ordinarily by the passive 

 contraction of the muscle itself; but that its 

 active contractions are mainly excited by vo- 

 luntary influence, allowance being made for 

 some slight action which may be produced by 

 the stimulus of sudden distension, as in other 

 circular muscles. Now, as a stimulus to sen- 

 tient nerves constitutes no necessary part of 

 any of these actions, it is probable that the 

 motor nerves of the sphincter have little or no 

 connection with the sentient ones; and, conse- 

 quently, that muscle is not usually excitable to 

 contraction by a stimulus appled to a sentient 

 surface. Hence, whenever the influence of the 

 will upon the lumbar portion of the cord is 

 suspended, this muscle ceases to act, whether a 

 mental or a physical stimulus be exerted. 



We have remarked before that all that is 

 shown by Dr. Hall's experiments on the horse 

 and on the turtle is that the spinal cord influ- 

 enced the sphincter only whilst it was in a state of 

 irritation consequent upon its division. There 

 probably was no real reflex action at all, and the 

 closure of the anus on the application of a stimu- 

 lus was probably only apparently due to that 

 cause, frequent contractions taking place in the 

 muscle in effect of the irritated state of the cord. 

 On the same principle, animals will exhibit 

 movements of voluntary character for some 

 time after decapitation, the continued irritation 

 of the cord acting as a stimulus. A bird thus 

 treated will fly for some distance, and with 

 considerable energy, and will flap its wings if 

 the cut surface of the cord be irritated. A fly 

 decapitated pursues its course for some way 

 immediately after the removal of the head; and 

 Walckenaer observed a singular fact respecting 

 the Cerccris ornutu, a wasp which attacks a bee 

 that inhabits holes : " at the moment that the in- 

 sect was forcing its way into the hole of the bee, 

 Walckenaer decapitated it; notwithstanding 

 which, it continued its motions, and, when 

 turned round, endeavoured to resume its posi- 

 tion and enter the hole."* The change in the 

 vesicular matter of the ganglia necessary for 

 the movements of the wasp in pursuit of its 

 prey, had already been excited by a powerful 

 stimulus of volition, which continued even 

 after the removal of the centre from which it 

 had emanated. Actions at first voluntary, 

 which by frequent repetition become habitual 

 and involuntary, are, no doubt, to be accounted 

 for by the persistence of that condition of the 

 vesicular matter which the will at first induced, 

 and to which the frequency of repetition <:ives 

 a character of permanence. Thus Habit is due, 



* Quoted in Wullur's Physiology, by Baly, vol. i. 

 p. 787% 2nd cd. 



as it were, to the fixation of a certain state of 

 vesicular matter it is the conversion of a men- 

 tal into a physical nervous action by frequent 

 repetition. 



So similar is the change which a physical 

 stimulus can excite in the grey 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 re- 

 semblance to the ordinary voluntary move- 

 ments. When a certain portion of the skin is 

 irritated, the animal pushes against the offend- 

 ing substance, as if trying to remove or dis- 

 place it. If the anus be irritated, both legs 

 are excited to action. It may also be observed, 

 that the same motions follow the same irrita- 

 tions of the skin. If, in a frog, the seat of 

 irritation be on the right side, the correspond- 

 ing 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 suppo- 

 sition that the same fibres are employed in the 

 execution of both. 



It must be kept in view, that, while this 

 hypothesis rejects the class of sensori-volitional 

 fibres which are supposed to 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 involuntary 

 motion ; and commissural fibres ; of which the 

 former only contribute to form the nerves. It 

 must not be supposed, however, that it is intend- 

 ed by this hypothesis to assume that the inter- 

 vention of sensation (i.e. the perception of an 

 impression by the mind) is necessary 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 

 imlt'/'titi/ait/i/, cause a change in the vesicular 

 mutter which shall stimulate the motor nt-ivt-s ; 

 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 enun- 

 ciation of a highly important proposition u:t!i 

 reference to nervous centres. It is this: ihat 

 all the centres which are connected to the brain 

 by commissural fibres, are thereby submitted 

 to, and brought into connection 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 af- 

 fecting them, affect the mind simultaneously. 



In voluntary actions, then, it may be stated 

 that, while the brain is the part primarily af- 

 fected, the mental impulse is also directed to 

 that portion of the cord upon which the required 

 action depends. 



In the developement of sensation the stimu- 

 lus affects the posterior horns of the grey matter 

 of the cord, which, from its commissural con- 

 nection with the brain, is in reality a part of 

 the sensorium. When the power of mint, i! 



