nerves, we see that the roots of both nerves originating on nearly the same level are affected, or the nerves lying behind, never those lying before the sensitive root to which the excitation was applied. Irritation of the optic nerve, for example, produces contraction of the iris -i.e., reflex from the optic to the oculo-motor nerve, intersensitivemotor movement from before backwards. In the spinal cord, the primary affected motor nerve is more or less on the same level with the root of the irritated sensitive fibre. When the reflex excitation extends itself in the spinal cord, it does so from behind forwards towards the medulla oblongata-never from before backwards. Excitation of a cutaneous nerve of the finger awakens reflex actions in the brachial plexus, next in the cervical plexus, the accessories, the vagus, &c., not in the dorsal or lumbar nerves. The irradiation first travels backwards after it has reached the medulla oblongata. 5. The law of threefold origin of reflex. The reflex motions which the excitation of a sensitive fibre produce can appear only and absolutely on three parts of the body, whether they are on one side or on both sides. (a.) Reflex appears in those motor nerves which lie more or less on the same level as the excited sensitive fibres. (b.) If the reflex appears in motor nerves at a distance from the sensitive fibres, these motor nerves are always such as arise in the medulla oblongata. This is exemplified in trismus after wounds of certain parts of the skin, hysterical laughings, convulsions. (c.) A reflex may occur in all the muscles of the body. The principal centre of radiation for this general reflex is the medulla oblongata. In proof of this, Pflüger adduces extensive pathological evidence. The theory of reflex action is that it is effected by multipolar ganglionic cells, but it is still undetermined whether there exists a special set of centripetal fibres whose function is to conduct excitations to the motor nerve-cells, or whether this is effected by the same fibre whose function it is to conduct sensitive impressions to the brain. Nor has it been shown how excito-motory acts are confined to certain muscles or groups of muscles, so as to produce that precision of result which we witness in all the ordinary reflex acts. Very interesting is the study of the agencies, psychical (the will, sleep) and physical (strychnia, opium, chloroform, &c.), which modify reflex action; but space forbids our entering upon these. 6. The spinal cord as a conducting organ. The labours of two contemporaneous physiologists have entirely revolutionized and remodelled our notions regarding the conduction of sensitive and motor impulses. To Brown-Séquard and Schiff we owe almost exclusively our present knowledge of the cord as a conducting organ. Since Sir Charles Bell discovered that the conductors of sensation and volition are distinct from each other from the brain to the periphery, it has been a problem to show how they are disposed of in the spinal cord. For many years the theory originally propounded by, and naturally following the discovery of Sir Charles Bell, that the posterior columns conduct sensory, the anterior motory impressions, was supported by Longet, and all but universally received by physiologists. Through the wonderfully exact researches of Schiff and Brown-Séquard, this * theory has been shown to be quite false. We propose in what follows to state what is now known regarding conduction in the spinal cord, adopting as a text the work of Schiff, which, in point of minute detail, is the most learned and precise work on the subject. And first, in reference to sensory impressions. The posterior columns are the only sensitive parts of the spinal cord. This fact was first fully recognised by Longet. But it was not known to him that these columns owe their sensibility to the branches of the nerve-roots which traverse them, and not to the proper longitudinal fibres of the columns. The discovery of this fact is specially due to Schiff, and it has since been confirmed by Brown-Séquard. Although these columus are the only sensitive parts of the cord, their complete section by no means arrests the conduction of sensation above or beyond the divided portion. Fodera, and afterwards Stilling and Van Deen, had noticed this fact in frogs, even after a considerable portion of the posterior column was removed. Section of the posterior columns produces hyperesthesia of the parts behind the section. The first to indicate this fact was Fodera, and its subsequent elucidation has been claimed respectively by Brown-Séquard and Schiff. That the grey substance conducts sensation is proved by dividing all but the grey substance. The conduction of sensibility after section of the posterior columns appears to take place in the grey substance only, as the anterior and lateral columns do not. in Schiff's opinion, conduct sensitive impressions, although Brown-Séquard, and, earlier, Schoeps, Calmeil and Budge, have claimed for the anterior columns the power of conducting such impressions. What parts of the grey substance conduct sensitive impressions? Van Deen was the first to observe that sensitive impressions are conducted as long as any part of the grey substance is left undivided. In order to prove the capability of conduction in the different parts of the grey substance, these parts must be isolated from each other, and each tested when it alone forms the sole bond of connexion betwixt the anterior and posterior parts of the body. Brown-Séquard believes that it is essentially the central part of the grey substance-i.e., the lateral masses, the bases of the anterior and posterior horns, and all the substance around the central canal-which is employed in the transmission of sensitive impressions.† Schiff is of opinion that no good grounds exist to support such a view, but that any part whatever is capable of conducting sensibility, and that so long as a mere bridge of grey substance is left to unite the anterior with the posterior half of the body, evidences of impressions made on the latter can be readily obtained. It is interesting to observe that the posterior horns can conduct sensitive impressions, as from the anatomical views of Bidder, referred to in the first part of this review, we should be led to suppose that they would not do so, were they composed, as Bidder and his pupils affirm, of cellular tissue only. Schiff has also observed that the smaller the amount of connecting grey substance left, the more slowly are impressions transmitted. This explains an observation made by * Lehrbüch der Physiologie des Menschen. Lectures, p. 23. * Cruveilhier, who remarked, in a case of paralysis of motion, that fifteen to thirty seconds elapsed ere the patient perceived irritations of the paralysed parts. The relation of the parts of the grey substance to the parts of the body behind them is such that every segment of grey substance conducts the sensibility of all, rather than of special parts of the body behind it. On the other hand, it must follow that impressions made on one small portion of the body are transmitted to the brain by conducting elements present in all parts of the grey substance of the spinal cord. This fact explains the retention generally of sensibility in many cases of partial softening, also why in hyperæsthesia or anaesthesia these alterations of sensibility are general and not partial. From personal experiment the writer can corroborate most of these observations on the conducting properties of the grey substance, and in connexion therewith it is interesting to recollect the anastomosing character of the nerve-cells. The grey substance is in itself not sensitive. Although this fact was remarked by Longet, he failed to observe that it is the conducting agent of sensibility, whilst Stilling, Eigenbrodt, and others who recognised the latter fact, did not recoguise its own want of sensibility. The first to perceive both properties was Schiff (1853). He terms the grey substance "æsthesodic" (from odos, way). Strychnine, which increases the reflex irritability of the grey substance, does not impart to it sensitive properties. As regards the power of conduction possessed by the posterior white columns, the experiments of Brown-Séquard and Schiff agree in proving that the white fibres are not capable of conducting, for any great length, sensitive impressions towards the brain, but that the transmission of such impressions, after it has traversed for a short distance the white substauce, is then transferred to the grey matter. This rule applies in both directions-i.e., the white matter conducts a certain distance backwards (towards the tail of the animal) as well as forwards. These experiments harmonize with what is known anatomically of the course of the posterior nerve-roots. One of the most curious results of Schiff's researches on the cord is that which relates to the condition which he terms analgesia. If in a rabbit every part of the spinal cord be divided, excepting the posterior columns, a peculiar state is induced, in which all parts behind the section are sensitive to a mere touch, but not to what in other circumstances would induce pain. In reference to this subject Schiff remarks: 66 Analgesia is a state which we observe in many persons during an advanced stage of chloroform inhalations, in which it perhaps always occurs, although it is often unobserved, because the patient is not sufficiently intelligent to give a clear account of his feelings. He accurately feels the hand of the surgeon which holds his limbs; he feels the impression of the knife as it is placed on the skin, but experiences no pain during division of the part; he feels the shaking during the sawing of the bone, and may even be busy with * Anat. Pathologique, xxxviii. p. 9. pleasant dreams which perhaps this very shaking may have called forth or interrupted. A patient who is undergoing tooth-extraction is quite aware that the instrument is pressed into his jaw; he feels the tooth loosening from the gum, but the pain is absent, and he has only the indifferent feeling of its being done. A similar condition has been often (and first by Beau) observed in paralysis. A Swiss physician, Vieusseux, who had a disease in the central nervous system, for a long time remarked that he could feel with one hand impressions of touch and the pulse, but compression or pricking of the hand produced no pain." Schiff then found that he could produce in rabbits a condition similar to that described by a rapid, but moderate bloodletting. This being done, every part of the spinal cord except the posterior white columns was divided. The sensibility of the parts behind being now tested, it was found that even a slight touch of the tail would cause the animal to raise its head, open its eyes, and move its ears, whilst the breathing, at the same time, became more hurried. If the tail, instead of being merely touched, was pinched until the nails reached the bone, none of these signs of disturbance occurs. From these and similar observations, Schiff concludes that the conductors of tactile sensibility are functionally and structurally distinct from those of sensibility to painful impressions; further, that these conductors reach the brain through the posterior white columns of the spinal cord; lastly, that each white column contains only the nerve-elements which convey the tactile sensibility of the corresponding half of the body, and that in this case the law of isolated conduction holds good, so that partial destruction of the white columns produces a lesion of sensibility in a corresponding part of the body. The question of a decussation of the conductors of sensitive impressions in the cord is one on which there is still much difference of opinion. The simplest experiment on the question is that of division of one half of the cord. According to Brown-Séquard, the invariable result of this experiment is hyperæsthesia behind the section, whilst on the opposite side sensibility is lost, or extremely diminished. He thence infers that there is a decussation of the conductors of sensitive impressions very nearly, if not absolutely, complete (p. 35). In regard to the loss of sensibility on the side opposite the section, Van Deen, Stilling, Valentin, and Schiff, have found that, in many cases, sensibility is not at all diminished, and in none wholly suppressed. Indeed, Brown-Séquard himself has found that sensibility frequently returns or remains on the side opposite the section, but gives as an explanation, that by a reflex action the muscles of the side of the section (which is in a state of hyperæsthesia) contract spasmodically, and thus produce pain. To this view Schiff raises objections which it would occupy too much space to enter into. Nor does he agree with Brown-Séquard as to the results of the other experiment on which the latter argues that a decussation of the sensitive conductors takes place. From a very great number of vivisections on all species of animals, as well as from the study of pathological cases, Schiff has come to the conclusion that the principal mass of grey matter serves to conduct sensitive impressions from either one side of the body or the other. To the extreme right of the grey matter a very small portion exists, which conducts sensitive impressions from the left side of the body only, and vice versâ. The grey matter conveys impressions in all directions; and the agents employed in this omnilateral transmission are the ganglionic cells. To meet Schiff's experimental results, anatomy has to show that on either edge of the grey matter to the extreme right and extreme left, a group of cells exists, the process of which is not connected with the sensitive root-fibres of the same, but of the opposite side. As far as we are aware, no observer has yet ascertained whether such a relation holds. Schiff also maintains that pathological cases favour his ideas as to decussations much more than they do those of Brown-Séquard. Several years ago, Von Bezold made an extended series of experiments on the question of decussation, and the result was equally adverse to Brown-Séquard's views.* All experiments on the conduction of motory impulses are much more difficult than those on sensation, principally because one must wait until the animal makes a voluntary movement. No irritation, save that of the will, can be relied on. Another source of difficulty lies in the greater delicacy and vulnerability of the parts of the cord which conduct movement, a very slight degree of compression serving to produce a paralysis of motion. Continuing our analysis of the experiments of Schiff, we find that in agreement with Van Deen, he believes that the anterior and lateral white columns transmit movement in the direction of the long axis of the cord. If in a frog all but the white anterior and lateral columns be divided, voluntary movements are still possible. In consequence of the difficulty of avoiding injury of these columns, the movements are always weaker than normal. In mammals, Schiff has also succeeded in observing voluntary movements when all but these columns were divided. As anatomy would indicate, and as pathology sanctions, these fibres do not go out of the cord through the motor roots, but join the cells in the anterior grey matter, which also serves for the conduction of movement. If the anterior and lateral columns are divided, after a time voluntary movements are strongly and harmoniously produced behind the section, so that the view of Longet, that the white anterior columns are the sole conductors of movement, is thus proved to be false. The question which next arises is-What portions of the grey substance conduct motor impressions? The answer is not such as former notions would lead us to expect, for Schiff found that when all the anterior parts of the cord were divided, and only a thin segment of the posterior grey substance (a segment so small that it could not be seen with the naked eye!) was left, spontaneous movements of the parts behind still took place. Other experiments showed that the central grey substance, as well as the posterior, conducts motory impulse, whilst experiments instituted to ascertain the direction in which conduction of motor impulse * Ueber die Gekreuzten Wirkungen des Rückenmarks: Zeitschr. für wissensch. Zoologie, ix. p. 307. |