CHAPTER X.

ACTION OF THE SPINAL CORD AS A CONDUCTOR.

Transmission of motor stimulus in the cord—Decussation of the motor conduct

ors of the cord-Decussation at the medulla oblongata—Decussation of the motor conductors in the cervical portion of the cord-Transmission of sensory impressions in the cord-The white substance of the posterior columns does not conduct sensory impressions-Action of the gray matter as a conductor-Probable function of the cord in connection with muscular coördination-Decussation of the sensory conductors of the cord-Summary of the action of the cord as a conductor.

IN treating of the functions of the spinal cord, both as a conductor and as a nerve-centre, we shall endeavor to discuss those facts only which are, it is to be hoped, either definitively settled, or are in accordance with what is at present known in anatomy, physiology, and pathology. The literature upon this portion of our subject is so extended and diffuse, that a full, critical analysis of the different experiments and views that have been presented since the observations of Magendie, in 1823, would inevitably complicate and confuse our description. We shall give citations, however, which will enable the reader to refer readily to the most reliable historical and controversial discussions upon this subject.'

1 Longet, in his treatise on physiology, gives a tolerably complete historical account of the numerous experimental researches concerning the functions of the cord as a conductor (Traité de physiologie, Paris, 1869, tome iii., p. 338, et seq.). The writings upon this subject by Brown-Séquard are very voluminous, and are scattered through numerous periodical publications, while many of his papers are controversial, and are reiterations of experiments and views previously pub

Transmission of Motor Stimulus in the Cord.—The antero-lateral columns of the cord, both the white and the gray substance, are entirely insensible to direct irritation, and conduct the motor stimulus from the centres to the periphery. This statement may be accepted, as the result of positive demonstration, with very little qualification.

If the posterior columns of the cord be divided or even removed for a certain length, the animal retains the power of voluntary motion intact. It is supposed by Dr. BrownSéquard that the white substance of the antero-lateral columns, in addition to its motor properties, takes a slight but well-defined part in the transmission of sensory impressions, and this idea is based upon experiments which seem to show that slight sensibility remains in the lower extremities after section of the posterior columns.' Such experiments, however, must be accepted with a certain degree of reserve, in view of the great difficulty of dividing the columns separately. If the white substance of the antero-lateral columns take any part in the conduction of sensory impressions, it is slight and unimportant. On the other hand, if the anterolateral columns of the cord be divided on both sides, the power of voluntary motion is lost absolutely in all parts supplied with nerves coming from the cord below the section.

It would be an interesting point to determine positively the relative importance of the white and the gray substance of the anterior columns in the transmission of motor stimulus; but this has thus far been impossible. We cannot with certainty divide the gray matter of the anterior columns completely and leave the white substance intact, nor can we divide the white substance without injuring the gray. As far as experiments go, however, they seem to show that lished. A list of his most important memoirs, with a short account of his experiments and conclusions, is given in the Journal de la physiologie, Paris, 1862, tome v., p. 641, et seq.

1 Brown-SéquarD, Expériences montrant que les cordons antérieurs de la moelle épinère servent à la transmission des impressions sensitives.―Journal de la physi ologie, Paris, 1858, tome i., p. 809.

transmission is not effected exclusively by the white substance, but that the gray matter plays an important part in this function. We shall refer, farther on, to the action of the gray substance in the transmission of sensory impressions.

It is evident, from anatomical facts as well as from the results of direct experimentation, that the fibres of conduction of motor stimulus pass from the brain to the anterior roots of the nerves, through the spinal cord, from above downward, and that there is no other medium for the transmission of the will to the muscles. Wherever the cord be divided, all the muscles supplied by nerves given off below the section are paralyzed. From the brachial enlargement of the cord, nerves of motion pass to the superior extremities, and the inferior extremities are supplied mainly by nerves coming from the lumbar enlargement. The di rection of these motor fibres in the cord itself has only been elucidated by experiments upon living animals. If the anterior columns alone be divided in the dorsal region, there is almost complete paralysis of the lower extremities. If the lateral columns be divided in this situation, without injuring the anterior columns, voluntary movements of the lower extremities are diminished, but are not abolished. If the anterior columns be divided high up in the cervical region, there is a diminution in the voluntary movements, but by no means so marked as when the section is made in the dorsal region; but if the lateral columns be divided in the upper cervical region, the paralysis is almost or quite complete.*

The experiments just cited clearly show that the situation of the chief motor conductors of the cord is different in the dorsal and in the cervical region. In the dorsal region, while conduction of the motor stimulus takes place through fibres contained both in the anterior and in the lateral

1 VULPIAN, Leçons sur la physiologie générale et comparée du système nerveux, Paris, 1866, p. 369.

2 BROWN-SÉQUARD, Physiology and Pathology of the Central Nervous System, Philadelphia, 1860, p. 46. VULPIAN, Système nerveux, Paris, 1866, p. 370.

columns, the transmission is mainly through the anterior columns, the lateral columns being much less important. In the cervical region, the conditions are reversed, and the conduction takes place chiefly by means of the lateral columns. Passing from above downward, therefore, the motor fibres are situated in the cervical region mainly in the lateral columns; but progressively, as they pass through the dorsal and the lumbar portions of the cord, these fibres change their location and are found chiefly in the anterior col

umns.

Recent observations have not sustained the old idea that the lateral columns of the cord contain fibres which preside specially over the movements of the thorax. The experiments of Vulpian upon this point are conclusive. If the lateral column be divided on one side at about the third or fourth cervical vertebra, there is considerable enfeeblement of the muscles of the thorax upon the corresponding side, but there is also partial loss of power in the limbs, which is more marked in the anterior extremity. This diminution in power in the thoracic muscles is such, that in ordinary tranquil respiration, the side corresponding to the section does not move; but in difficult respiration, or in crying, the movements are very marked.

Decussation of the Motor Conductors of the Cord.—Wellestablished anatomical and pathological facts show conclusively that there is a complete decussation of the motor conductors of the cord; so that the stimulus of volition generated in one lateral half of the brain always passes to the opposite half of the body. If a lesion occur in the brain. upon one side, so as to produce total paralysis of motion, the opposite side of the body is paralyzed, while voluntary motion is absolutely intact on the side corresponding to the injury. In the anterior pyramids of the medulla oblongata,

1 VULPIAN, Système nerveux, Paris, 1866, p. 371.

the decussation of the fibres is easily demonstrated anatomically. In view of these facts, concerning which there is no difference of opinion, it only remains to show by physiological experiments that decussation actually takes place at the medulla oblongata, and to submit to the same method of inquiry the following important question: Assuming that crossing of motor fibres takes place at the medulla, is this the sole seat of decussation of these fibres, or does it also exist in certain portions of the cord below?

The question of decussation at the medulla oblongata is easily answered. In the first place, we have the crossed action in hemiplegia and the easy anatomical demonstration of the decussating fibres. The experimental confirmation of these facts is not so simple, for the reason that animals survive operations upon the medulla oblongata for a very short time. As far as can be learned, however, from the latter mode of inquiry, the conclusions drawn from anatomy and pathology are fully sustained. If the medulla be exposed in a living animal, and "if a section is made longitudinally just at the place of the decussation of the anterior pyramids, so as to divide completely all of the decussating elements, we find that, although the animal lives some time after the operation, it has no voluntary movement at all in any of the limbs, which are almost always the seat of convulsions."

The question of decussation of motor fibres in the cord. itself is one which can be settled only by physiological experiments, as the course of the decussating fibres, if they exist, cannot be demonstrated anatomically. It is remarkable that Galen submitted this point to experimental investigation, by dividing the cord longitudinally in the median line in the lumbar region. This operation was not followed by loss of voluntary power in the lower extremities, showing that the motor fibres do not cross the median line, at

1 BROWN-SÉQUARD, Physiology and Pathology of the Central Nervous System, Philadelphia, 1860, p. 49.