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member has entirely lost its sensibility. Neither pinching, pricking nor cutting of the skin or other parts of the left limb, neither burning, nor the application of caustics nor of galvanism, seems to excite the least sensation of pain. Sometimes, it is true, it has happened to me to recognize the existence of some signs of pain in crushing the sciatic or the crural nerve of the left side; but most frequently I have found in such cases, at the autopsy made after the cord has been hardened in alcohol, that a small part of the right lateral half of the cord had not been divided. On the contrary, in the right posterior member, sensibility is everywhere exalted, so that a slight pressure on any part of this member suffices to produce indications of pain. All causes of pain act with much greater intensity than they do in animals of the same species in a normal condition; such as galvanism, the application of ice, of flame, or of caustics, mechanical irritations (pricking, pinching, cutting), &c. After having fully assured myself of the existence of hyperesthesia in the right posterior member (that corresponding with the side of the hemi-section of the cord), and of the loss of sensibility in the left posterior member, I divide the nerves of the cervical and brachial plexus, in order to prevent the movements of those members, whether reflex or otherwise, from taking any share whatever in the production of pain, when the right posterior member is irritated. After the performance of this operation, I find that the hyperesthesia has not disappeared in that member. It is diminished, like the sensibility of the skin of the thorax; but I repeat that its existence is incontestible. The animal cries out when its foot is pinched, burned, or galvanized, &c."*
The results of longitudinal sections of the cord, carefully made so as to divide as little as possible except the median commissure, entirely accord with those obtained from transverse hemi-sections. When the operation has been performed through the entire extent of that part of the cord which gives origin to the nerves of the posterior limbs, and when the two separated halves have been themselves but little injured, the very striking result is obtained of the entire extinction of sensibility in both limbs, without any considerable reduction in the power of voluntary movement; a result the more remarkable when it is borne in mind that ordinary injuries of the spinal cord produce a greater effect on the mobility than on the sensibility of the parts behind the seat of injury. As it seems impossible to impute this extinction of sensibility to the injurious effect of the section on the lateral halves of the cord, since this would have manifested itself in the like diminution of the motor power, there seems to be no other possible explanation of the fact, than the one offered by M. Brown-Séquard, namely, that the section has divided the commissures by which the entire aggregate of the conductors of sensory impressions from each lateral half of the body passes over into the opposite half of the cord.
The proof seems to be completed by the combination of both methods of experiment, the longitudinal section, and the transverse hemisection. If a longitudinal section be made in the cervico-brachial enlargement of the cord, so as to divide its two lateral halves, it is found that sensibility is entirely destroyed in the anterior extremities, whilst the sensibility of the posterior extremities is rather augmented than diminished, showing that no injury has been done to their conductors. But if either of the separated halves be then divided trans
* Journal de la Physiologie, Janv. 1859, p. 66.
versely, there is found to result a complete anaesthesia of the opposite posterior extremity, with a further increase in the sensibility of the hinder limb of the same side.-We cannot see how the results of this experiment can possibly be explained on any other hypothesis than that of the decussation of the sensory conductors in the cord itself. It is true that such decussation cannot be anatomically demonstrated. Of the fibres of the posterior roots which enter the cord transversely, some appear to cross through the central commissure; but these seem to come into connection with the anterior roots of the opposite side; and there are other reasons (which will be presently adverted to) for the belief that these transverse fibres are chiefly concerned in exciting reflex actions. On the other hand, the fibres which at first pass longitudinally, whether upwards or downwards, in the posterior columns, and which seem to be the chief conductors of sensory impressions, cannot be shown to undergo any decussation; after their entry into the grey substance of the cord, their course can no longer be determined, so that it cannot be affirmed that they do not cross; and in so far as the vesicular rather than the fibrous substance of the cord is the conductor for sensory impressions, their passage from one side of the cord to the other, or in any other direction, might take place without any structural indication of their course. This, however, may be specially noticed, that all the phenomena indicative of this decussation are opposed to the doctrine that the posterior columns are the chief conductors of sensory impressions; since it can be fully proved anatomically that those columns do not decussate.
In drawing inferences as to the effects of transverse hemi-sections of the spinal cord, upon the sensibility of the parts behind, it is necessary to ascertain in every instance that the operation has been effectually performed; for if, in his desire to avoid cutting too far, the operator leave but a small part of the anterior column and of the central grey substance undivided, he will meet with very decided indications of sensibility in the limb of the opposite side. It is wonderful, says M. Brown-Séquard, how small is the quantity of grey matter which, being left undivided, may transmit sensory impressions. And it is also necessary to bear in mind that the decussation is not equally complete in all animals; for whilst in Mammals (including Man, as pathological evidence indicates) a complete transverse hemi-section entirely extinguishes the sensibility of the opposite limb, the extinction is not by any means so complete in Birds and Reptiles; and this, together with the occurrence of pain as a result of reflex muscular actions, is the explanation offered by M. Brown-Séquard, satisfactorily as we think, of the apparently contradictory results obtained by M. Chauvean of Lyons, who has been the chief opponent to his theory of decussation.* In order to ascertain how far the sensory conductors pass longitudinally through the cord before they decussate, M. Brown-Séquard has made the following experiment:
"If we divide transversely a lateral half of the cord in two places, so as to * A full discussion of M. Chauveau's objections will be found in the Journal de la Physiologie, Janv. 1858, pp. 176–189.
have three pairs of nerves between the two sections, we find that the middle pair has almost the same degree of sensibility as if nothing had been done to the spinal cord, while the two other pairs have a diminished sensibility, the upper one particularly in its upper roots, and the lower one in its lower roots: which facts seem to show that the ascending fibres of the upper pair, and the descending fibres of the lower one, have been divided before they made their decussation. If there be only one pair of nerves between two sections, its sensibility is then almost entirely lost, as then the transversal fibres are almost alone uninjured (most of the ascending and descending being divided), which fibres are employed for reflex action, and hardly for the transmission of sensitive impressions. After having divided transversely a lateral half of the spinal cord in the dorsal region, if we divide this organ longitudinally, so as to separate its lateral halves from one another, and at a right-angle with the transversal section, we find that sensibility persists in the segment partly separated from the rest of the cord, if it is not more than two inches long, in a large mammal, whether the longitudinal section has been made below or behind the transversal one, or above or before this transversal division. If the longitudinal section is more than two inches long, it is not sensitive in all its length. When there are three pairs of nerves attached to it, the one nearest the transversal section is hardly able to give slight sensations; the next is a little more sensitive, but much less than in a normal condition; and the third is very sensitive, though not so much as others on the same side and behind it. With a segment attached to the cord by its upper extremity, similar results are obtained; and it seems certain, both from these facts and from many others which it is not necessary to mention, that the decussation of the conductors of the sensitive impressions in the spinal cord, whether they are at first descending or ascending, takes place at a short distance from the point of insertion of the posterior roots."
We come now to consider the evidence adduced by M. BrownSéquard in support of his position, from Human Pathology; and this is so cogent as to appear to us absolutely conclusive,-until, that is, we are made acquainted with opposing evidence equally unexceptionable. One of the most interesting cases cited by him, which is unfortunately too long to be extracted in full, is that of a man who received a sword-wound in the back, the position and direction of which were such as to indicate that the sword-cut divided the whole of the left half of the spinal cord with the exception of a part of the anterior column, whilst it may have divided a part of the right posterior column, leaving the remainder of the right half of the cord uninjured, save by the pressure of the instrument and the effusion of blood. Now the first effect of this injury was to produce paralysis of motion in both legs; but the patient soon came to be able to move his right leg, and in a few weeks he began to recover voluntary power over his left leg. On the other hand, from the very first there was a decided hyperesthesia of the left leg, and a great diminution in the sensibility of the right; the hyperesthesia diminished as the motor power was recovered; but a deficiency of sensibility remained in the right leg even three years subsequently, when the motor power had been completely recovered in both. Although in this important case the sanction of a post-mortem examination was deficient, yet its phenomena were so precisely accordant with those which present themselves in experiments on animals, and were so inexplicable on the
ordinary hypothesis by any supposition as to the nature of the injury which should be consistent with the course and direction of the wound, that we think M. Brown-Séquard is fully justified in adducing it as furnishing very important evidence in his favour, more especially as its phenomena were very carefully noted by intelligent observers. The evidence deficient in this case is supplied by several others, in which injury to one side of the spinal cord by the pressure of tumours, hemorrhagic effusion, or disorganizing disease, produced loss of motor power in the parts below on the same side, with retention of sensibility or even hyperesthesia of that side, and with anesthesia of the opposite side. Another series of cases is adduced to prove that any lesion of the nervous substance which takes place in the medulla oblongata above the decussation of the pyramids, affects both motion and sensation on the opposite side of the body; and this whether the lesion be only just above the pyramidal decussation, or whether it be much higher up; so that they leave no doubt that if the decussation of the sensory conductors does not take place in the same locality as that of the motor conductors, it must be below rather than above this. And thus we have a most important basis for the determination of the seat of the lesion on which paralysis depends; for if there should be loss of motor power on one side, without any diminution or even with an increase of sensibility on that side, the sensibility of the other side being decidedly diminished, it seems certain that the seat of the mischief is in some part of the spinal cord; while if the loss of motor power and of sensibility should present themselves on the same side, the lesion must be in the medulla oblongata, or in its upward prolongation; whilst if there should be partial loss of motor power on both sides, with loss of sensibility on one side only, this may be presumed to depend upon a lateral lesion of the lower part of the medulla oblongata on the opposite side to that of the anesthesia, and at such a level as to involve some of the motor fibres which have already crossed, together with others which have not yet made the passage.
IV. With regard to the channel along which Motor Power is transmitted down the Spinal Cord from the Encephalon to the anterior roots of the Spinal Nerves, we do not find that M. Brown-Séquard's researches have much affected what was previously received as probable truth. Although Sir C. Bell had originally regarded the anterior columns as the conductors of volitional influence, yet he had himself been led in his later years, by the study of the connections of the anterior roots of the spinal nerves with the lateral columns, to regard the latter as participating in this function; and the term "anterolateral columns" was frequently employed in this country, to designate the part of the spinal cord which was supposed to be subservient to it. The statement of Dr. John Reid, confirmed by other eminent anatomists, that the fibres of the anterior pyramids pass backwards in their course downwards, so as to connect themselves with the lateral rather than with the anterior columns, gave additional strength to this view; and the notion that the anterior columns are the exclusive or even the special conductors of motor power has seemed to receive a
complete disproof from certain pathological phenomena, although other pathological phenomena, together with the results of experiment, have appeared to justify the conclusion that they have no small share in its transmission. According to M. Brown-Séquard, transverse division of the posterior columns in the dorsal region, has but little effect upon the voluntary movements of the animal; whilst after transverse section of the whole spinal cord except the posterior columns, there appears to be a complete extinction of the power of voluntary movement in the parts behind the section. And thus, notwithstanding the admitted effect of extensive lesions of the posterior columns in impairing volitional power over the limbs, he thinks that we are justified in affirming that this part of the spinal cord is not directly concerned in the transmission of the mandates of the will to the muscles. If, however, the transverse section be carried not merely through the posterior columns, but also through the posterior horns of the central grey substance and a part of the lateral columns, he states that there is an evident though very slight diminution of voluntary power; whilst if the whole spinal cord except this part be divided, the power of voluntary movement seems completely destroyed. The first experiment would seem to show that either in the posterior horns of grey substance, or in the lateral columns, there exist some conductors of volitional power; whilst from the second it would appear that these conductors are in themselves insufficient for the stimulation of the muscles to action. The further forward the transverse section is carried, the greater is the reduction of voluntary power; if it divide the whole of the central grey substance, so as to involve rather more than the posterior half of the cord, the animal can hardly move its posterior extremities; and if, in addition, it divide the anterior horns of the grey substance, the greater part of the anterior columns being left entire, the loss of volitional power over those limbs seems to be complete. Yet if the anterior columns be alone divided in the dorsal region, there seems to be a complete extinction of voluntary power in the hinder limbs. Thus if the former result were taken alone, it would seem to justify the conclusion that the central grey substance and the lateral columns are the principal if not the sole channels of motor power, the anterior columns not being able by themselves to convey the least influence from the will to the muscles; whilst if the latter result were accepted to the exclusion of the other, it would lead to the opposite belief,-namely, that the anterior columns have this function to themselves, and that the remainder of the cord is incapable of sharing in it. From other experiments it appears that any injury to the central grey substance, and that a deep injury to the lateral columns, in the dorsal region, always produces a diminution in the voluntary movements of the posterior extremities.
"We do not see any way," says M. Brown-Séquard, "of explaining these various results, except in admitting, what seems to be proved by thousands of pathological cases and vivisections, that voluntary movements require very powerful excitations from the nervous system upon muscles, and that when one-half or one-third of the normal amount of excitation is missing, what remains is insufficient."