PHYSIOLOGY OF THE NERVOUS SYSTEM. 



722E 



channels for voluntary, emotional, or strictly 

 ji/ii'Xical impulses to motion. 



The mechanism of a voluntary action in 

 parts supplied by spinal nerves would be, 

 according to this hypothesis, as follows: 

 The impulse of volition, excited primarily in 

 the brain, acts at the same time upon the grey 

 matter of the cord (its anterior horn), and 

 through it upon the anterior roots of the nerves 

 implanted in it. This grey matter, in virtue of 

 its association with the brain by means of the 

 anterior pyramids, becomes part and parcel of 

 the organ of the will, and therefore as distinctly 

 amenable to acts of the mind as that portion 

 which is contained within the cranium. If we 

 destroy the commissural connection with the 

 1 train through the pyramidal fibres, the spinal 

 cord ceases to take part in mental nervous ac- 

 tions ; or, if that connection be only partially de- 

 stroyed, that portion of the cord which the injured 

 fibres had associated with the brain is no longer 

 influenced by the mind. Again, if the seat of 

 volition in the brain be diseased, the cord or part 

 of it participates in the effects of the disease as far 

 as regards voluntary actions. That it is not too 

 much to ascribe such power to the pyramidal 

 fibres appears reasonable, if we consider how 

 the fibres of the corpus callosum, and perhaps 

 other transverse commissures, so connect the 

 hemispheres and other parts of the brain that 

 the separate divisions of a double organ act 

 harmoniously so as to excite but a single train 

 of thought, or, conversely, that two impressions 

 from one and the same source on a double 

 sentient organ are perceived as single by the 

 mind. 



An objection to this explanation will readily 

 be raised- namely, that the excitation of the 

 anterior horn of the grey matter, in the way 

 stated, does not explain the remarkable power 

 which the will has of limiting its action to one 

 or two, or a particular class of muscles. To 

 this, however, it may be replied that there can 

 be no reason for denying to the mind the 

 faculty of concentrating its action upon a par- 

 ticular series of the elementary parts of the 

 vesicular matter, or even upon one or more 

 vesicles, if we admit that it. can direct its 

 influence to one or more individual fibres, as 

 the advocates of the first and second hypotheses 

 do. If, indeed, we admit the one, we must 

 admit the other; for whether the primary exci- 

 tation of a fibre take place in the encephalon 

 or in the spinal cord, the part first affected 

 must probably be one or more vesicles of grey 

 matter. 



The series of changes which would develope 

 a sensation admits of the following explana- 

 tion according to this hypothesis: A stimulus 

 applied to some part of the trunk or extremities 

 is propagated by the sensitive nerves to the 

 posterior horn of the grey matter of the spinal 

 cord, and from the junction of this part with 

 the brain, either through the direct continuity 

 of the vesicular matter of the cord with that of 

 the centre of sensation, by the olivary column, 

 or through longitudinal commissural fibres, 

 analogous to or even forming a part of the an- 

 terior pyramids, this is simultaneously affected. 



To this, likewise, it will be objected that the 

 limitation of sensation is not sufficiently ex- 

 plained. But the reply is obvious ; the intensity 

 and kind of sensation depend upon the nature 

 of the primary stimulus at the surface, the 

 extent upon the number of fibres there stimu- 

 lated. Wherever these fibres form their proper 

 organic connection with the vesicular matter, 

 that matter will participate in their change to 

 an extent proportionate to the number of fibres 

 stimulated, and with an intensity commensurate 

 with the force of the primary stimulus. It is 

 not necessary to the developement of sensation 

 that the fibre stimulated should be implanted 

 directly in the brain; if it be connected with 

 this centre through the medium of vesicular 

 matter or through commissural fibres, all 

 the conditions necessary for the developement 

 and propagation of nervous force would appear 

 to be fulfilled. It must not be supposed, 

 however, that in making this statement we 

 mean to assign the spinal cord to be the seat of 

 sensation ; all we assert is, that the posterior 

 horn of the grey matter, as being the part in 

 which the sensitive roots are implanted, is the 

 seat of physical change excited by the stimulus 

 applied to the sensitive fibres, which change 

 must be perceived by the mind before true 

 sensation can be produced. In fine, by the 

 union of the posterior horns of the spinal grey 

 matter with the vesicular matter of the brain, 

 they become a part of the centre of sensation 

 so long as that union is unimpaired.* 



This hypothesis offers an explanation of the 

 hitherto unexplained phenomenon of impaired 

 sensation on that side of the body which is 

 opposite to the seat of cerebral lesion. If we 

 regard the anterior pyramids as commissures 

 between the sensitive as well as between the 

 motor portions of the cerebro-spinal centre, it 

 will be obvious that the posterior horns of the 

 spinal grey matter on the right side will be 

 associated with the left centre of sensation, and 

 vice versa. 



And we gain, moreover, an explanation of 

 the almost universal association of sensation 

 with reflex or physical nervous actions. The 

 excitor nerves of these actions being the same 

 as the sensitive nerves, the impression con- 

 veyed by them is calculated at once to excite 

 motion and sensation. The controlling influ- 

 ence of the will prevents many of the sensitive 

 impressions made through the spinal cord from 

 developing corresponding movements. And 

 this controlling influence is best explained by 

 this hypothesis, for as it admits no other motor 

 nerves connected with the cord but those over 

 which the will can exert an influence, it follows 

 that such mental influence, if more powerful 

 than the physical stimulus which the sentient 

 nerves convey, may prevail over it and neu- 

 tralize its force. On the other hand, under 

 certain conditions of great physical excitation, 

 (exalted polarity,) physical changes overcome 



* In all discussions relative to sensation it should 

 be kept in view that true sensation involves a men- 

 tal act, namely, the perception of a physical impres- 

 sion, and of (he concomitant physical change in 

 the nervous matter. 



