It is very generally admitted that the only channel by which the will can influence the spinal cord is through the fibres of the anterior pyramids of the medulla oblongata, the greater number of which decussate each other along the median line. But it is in the highest de gree improbable that these fibres, occupying so small a space as they do, should form the ag gregate of the volitional fibres (still less of the sensori-volitional fibres) of the trunk and extre mities. The whole of these fibres (of both sides) collected together would scarcely equal in bulk the anterior portion of one of the antero lateral columns of the spinal cord.
It has been affirmed that much support is given to the excito-motory hypothesis by Dr. Carpenter's and Mr. Newport's supposed de monstration of the two sets of fibres in the Arti culata. But these observations are far from deserving the name of demonstration. The inferences from them are derived from the apparent direction of certain fibres, and not from any actual tracing of them by dissection or by microscopic inspection. The observa tions, too, have been made with low powers, which are very insufficient for determining the precise disposition of the fibres and their rela tion to the vesicular matter of the ganglia.
These writers affirm that the longitudinal -fibres of the ganglionic chain of Articulata pass up to the brain and constitute the sensori volitional fibres, whilst other fibres pass in a transverse direction and are implanted in the ganglia. Were this the case, it might reason ably be expected that the brain would be the largest of the ganglia as containing the sum of the sensori-volitional fibres of the whole body. But let any one compare the size of the cere bral ganglia of the scorpion (as figured by Mr. Newport*) with the size of the animal and that of its cord, and it will be evident to him how disproportionately small such a centre is to the number of sensori-volitional fibres which must be distributed over so large a sur face and to so many muscles. Anatomy, how ever, offers no objection to the hypothesis that the roots of the nerves are implanted in the ganglia, and that the longitudinal fibres act as commissures between different segments (adjacent and remote) of the cord.
Neither do Mr. Newport's experiments on the myriapods and other Articulata throw any new light on the question of the existence of two orders of fibres; nor do they add anything to our knowledge beyond the important fact that actions take place in certain Invertebrata after decapitation, which are of the same nature with those which occur in Vertebrata after a similar mutilation. The rnechanisnz of these actions
has not been at all elucidated by these expe riments.
The excito-motory hypothesis is sufficient for the explanation of the movements of decapi tated animals, of parts in connection with small segments of the spinal cord, of limbs paralysed to sensation and voluntary motion from dis eased brain or spinal cord. But there are two phenomena familiar to those who observe dis ease with care, whidi cannot be explained by it ; these are the movements which may be ex cited by mental emotion in limbs paralysed to the influence of the will, and the total paralysii of the sphincter ani, which frequently accom panies diseased brain, whilst at the same time the limbs ate only affected to a partial extent or not at all.
Cases occur sometimes in which hemiplegia arises from an apoplectic clot, or other destruc tive lesion in one hemisphere of the brain. The arm and leg, or either of them, are completely removed from the influence of the will ; yet occasionally, as the effect of some sudden emo tion, fear, joy, surprise, the paralysed limb is raised involuntarily. Even so slight a cause as yawning (an act of emotional kind) will excite the palsied limb. Every time the patient yawns the arm will be raised involuntarily.
Such phenomena as these receive no ade quate explanation from the excito-motory hypo thesis. Mental emotions probably affect some part of the brain ; if the only communica tion between the brain and the limbs be by fibres of sensation and volition, it is impossible to understand how the emotional influence could be conveyed to them through a channel which has long been interrupted. If we are to adopt the excito-motory hypothesis, it will be necessary to suppose with Dr. Carpenter the existence of certain emotional fibres to explain the phenomena of this particular case. But it is difficult to admit the existence of three orders of fibres in each muscle, which, to be effective, must have the same relation to the component elements of the muscle. It is impossible to imagine how each order of fibre should comport itself with reference to the other two, so that their actions may not interfere. Nor can any one fail to perceive that the emotional fibres must be inhnitely less frequently employed than the others, and in some individuals so seldom called into action as to be greatly ex posed to the risk of atrophy for want of use.