TRANSVERSE TRANSMISSION.

299

a considerable degree, accomplished through the vesicular substance, the quality of which, in this respect, has been explained in the preceding chapter. But, besides this, the exterior fibrous structures possess a like function, correspondingly as they are connected with the motor or sensory roots of the nerves, the anterior columns being motor, and the posterior apparently sensory.

The spinal cord not only permits the passage of influences in its longitudinal, but also in its transverse direction. This is what Transverse might be anticipated from the structure and functions of the transmission of cells of its gray interior. If the cord be cut half through in

influences.

a given place, and again be cut half through on the opposite side, at a little distance above or below, impressions may be conducted through the intermediate portion, the vesicular material being then their only channel.

In a memoir on the distribution of the fibres of the sensitive roots, and on the transmission of impressions in the spinal cord, Dr. Brown-Sequard Brown-Sequard, referring to the two theories entertained at on the conducpresent-1st. That sensitive impressions reaching the cord tion of the cord. pass in totality to the brain along the posterior columns; 2d. That such impressions so arriving pass directly to the central gray substance, which transmits them upward-offers reasons for supposing that both these theories, and especially the first, are contradicted by facts.

It is his opinion that sensitive impressions reaching the cord pass in different directions, some ascending, others descending, but both going in part by the posterior columns, and in part by the posterior gray horns, and perhaps by the lateral columns, to penetrate, after a short distance, the gray central substance by which, or in which, they are transmitted to the brain.

He also shows that sensitive impressions of one lateral half of the body are transmitted principally in a crossed manner, that is to say, that they follow more particularly the opposite half of the cord to reach the brain; that the decussation of the conducting elements for sensitive impressions is not made, as is commonly said, at the anterior extremity of the pons; that the gray substance does not possess the property of transmitting sensitive impressions in every direction, as some have supposed; that most, if not all the conducting elements for sensitive impressions decussate in the spinal cord, the decussation occurring in part almost immediately on their entry into the cord, but that a few make their decussation at a certain distance above the point of entry, the majority, however, descending in the cord, and making their decussation below the point of entry; that if there are conducting elements for sensitive impressions which ascend throughout the entire length of the cord to make their decussation in the brain, their number must be very small;

and that alterations capable of producing a paralysis of sensibility, and situated upon any point of a lateral half of the cerebro-spinal axis, always produce a paralysis of sensibility on the opposite half of the body, and that there is no difference between the brain and the spinal marrow in this respect.

Analogy with ventral cord

of articulata.

Thus constructed, the spinal cord, as we shall presently show from Dr. Carpenter, evidently agrees with the gangliated ventral cord of the articulata, each portion of it from which a pair of nerves is given off representing each ganglion of that ventral cord, the difference in the two structures being, that in the spinal column the ganglia are commissured, so as to form, in appearance, one continuous mass, and agreeably to this view of its construction are the circumstances under which its enlargements occur. In those animal forms in which the entire trunk is concerned in locomotion, as in snakes and eels, the cord is nearly cylindrical; but as soon as special members for locomotion are developed, a corresponding increase of diameter is observed. Thus, in birds, the ganglionic enlargement corresponds with the region from which the nerves for the wings are given off; but in that tribe, as in the ostrich, the mode of locomotion of which is by the legs rather than by the wings, a corresponding posterior enlargement occurs. The same observations may even be more distinctly made during metamorphoses; thus, in frogs, while they are in the tadpole state the spinal cord is cylindrical, but bulging ensues in it anteriorly and posteriorly as soon as the anterior and posterior members are developed.

The translation of impressions which have been brought along the Reflex action centripetal fibres into motions, the exciting influence of which of the cord. is conveyed along the centrifugal fibres, includes what is understood as the reflex action of the spinal cord as developed by Dr. Hall. Its essential condition is its independence of the agency of the brain, and therefore unconscious nature. As general examples may be mentioned the movements which occur in swallowing; for after the food has been carried by voluntary action into the fauces, its passage onward to the stomach is perfectly involuntary. In like manner, the introduction of air into the lungs in ordinary respiration is involuntary; for though it may be, to a certain extent, under the control of the will, yet that extent is limited, a necessity for the motion presently arising, which soon becomes uncontrollable. The action of the valvular arrangements at the cardiac and pyloric orifices of the stomach, and the constant contraction of the sphincter ani, are farther illustrations. To these may be added those impulsive movements which we instinctively make on the approach of danger or in the act of falling, and perhaps, too, automatic walking, as we go from place to place in a state of mental abstraction, paying no attention to the course we take.

The cord is to be regarded as a longitudinal series of simple automatic nerve arcs, or, as we have termed it, a multiple automatic Automatic acarc. Each segment of it has therefore an independent action tion of the cord. of its own, but can conspire with its neighbors or be influenced by the

Fig. 144.

a

B

b

brain, by means of its commissural fibres, an arrangement of which numberless interesting instances might be furnished. The one represented in Fig. 144, which is from the cord of spirostreptus, may, however, suffice: A, under surface of a portion; B, upper surface; a, inferior longitudinal fibres; e, superior longitudinal fibres; f, fibres of re-enforcement, seen also at b and c; g, commissural fibres, seen also at d.

The power which the cord displays in this simple action is most strikingly seen when it is cut off from its cranial connections. The decapitated frog props himself up stiffly on his legs, and, if his cutaneous surface be irritated, exhibits antagonizing motions; such motions are all of the reflex character, and are commonly much more strikingly seen in cold than in warm-blooded animals; but even in man precisely the same results are witnessed during periods of the suspension of the activity of the brain, as, when the palm of the hand of a sleeping child is touched with the finger, the finger is at once grasped. As above stated, this reflex function of the cord is therefore independent of the brain, though the brain can control it, and this the more perfectly the higher the organization of the animal. independent of Breathing can go on, whether we pay attention to it or not, but we can arrest it if we choose for a time; and since in man this introduction of air is incidentally used for very refined purposes, by voluntary exertion we moderate or regulate it, as in the production of musical sounds in singing or of articulate sounds in speech.

Reflex action

the brain.

and cerebral

In a general way, there is not much difficulty in distinguishing between simple actions of the cord and those in which the brain Distinction beis participating. In the former, no weariness or fatigue is ever experienced; in the latter it is; and perhaps, even in action. these last, involving voluntary muscular action, though the control is to be attributed to the brain, the source of the force is in the cord.

These normal phenomena which the cord displays become greatly ex

302

RELATIONS OF THE SPINAL CORD AND BRAIN.

Increase of aggerated in certain conditions of disease, as, for example, in spinal action. tetanus, in which the slightest peripheral irritation may be followed by violent convulsive movement, or the same occurs by the agency of powerful poisonous substances, as strychnine. In these cases the action may be either limited simply to the cord, as in the tetanus brought on by opium in frogs, or the brain may be involved in it, as in cases of hydrophobia, in which the sound or sight of water, operating through the cerebrum, will produce spasmodic convulsions.

From the facts presented by the lower animals, it may be inferred that the spinal cord does not act as a single organ, but rather should be regarded as a collection of ganglia, special duties being discharged by special parts of it.

Connection of

brain.

With respect to the commissural action of the spinal cord, reference has already been made to the structural connection between the the cord and cord and the nervous regions above it, and in referring to the old anatomical doctrine that each of the spinal nerves is connected by continuous fibres with the brain, due weight has been given to the fact that the cord does not increase in thickness as it approaches the brain, but that its bulgings correspond to the regions from which it is necessary that an unusual supply of nerves should be given off. The force of this argument is, however, considerably diminished when we recollect that the nerve-tubes are by no means of uniform diameter, but are doubly conical in shape. Even, therefore, with a diminished diameter of the spinal cord, there might be an upward continuation of spinal fibres, the diameter of which is becoming less and less; and this seems to be rendered more likely from the analogy of the structure of the ventral cord of the articulata, in which fibres are sent to the cephalic ganglia for the purpose of establishing a communication between them and the roots of the nerves. But, however that may be, there can be no question of the influence of the brain over spinal action, and this, of course, implies structural connection of some kind-an intercommunicationwhich, if it does not take place solely through the white columns, must take place through the gray material. It is, however, important to observe that the gray material has no direct communication with that of the cerebrum, but, passing through the optic thalamus, ends in the corpus striatum, extending therefore in one continued mass through the cord, and terminating in that ganglionic organ. By one or both of these channels, white or gray, the impressions which are made upon the spinal sensitive nerves are presented to the brain, and in a similar manner the influences which produce voluntary motions are transmitted down. A section of any part of the spinal cord at once incapacitates the sions of the will from acting upon the parts beyond, the motions of which become therefore purely automatic, though the parts above still

Effect of le

cord.

FUNCTIONS OF THE SPINAL CORD.

303

display their customary phenomena. These effects are sometimes instructively witnessed in man when lesions of the cord have occurred through disease.

the cord.

If the view that has been presented respecting the continuation of fibres from the cord to the brain be correct, these fibres dis- Motor and sencharge a commissural duty. This would lead us to sup- sory tracts of pose that there is a correspondence between the functions of the columns of the cord and those of the roots of the spinal nerves, the anterior columns being motiferous, or in unison with the motor root of the nerves, the posterior being sensiferous, or in unison with the sensory root of the nerves. Agreeably to this, if the anterior columns be irritated, motions are excited in all those parts which are supplied with nerves beyond the irritated point; and if the posterior columns be irritated, in like manner pain is experienced. In this instance, however, a certain amount of motion is occasionally observed, but this has commonly been explained by referring it to reflexion within the cord. It has also been observed, as strengthening these views, that if the posterior columns be irritated after complete section of the cord, the result will depend on which of the cut portions be disturbed; if it be the lower, there will be no effect. An examination, under the same circumstances, of the anterior columns, demonstrates that, if the upper section be irritated, there is no effect produced; if the lower, there are convulsive movements of the parts supplied with nerves beyond.

From these results we should infer that the physiological functions of the anterior and posterior roots of the spinal nerves are participated in by the anterior and posterior columns of the cord, and might therefore expect that those functions would be continued in the higher distribution of the columns above the medulla oblongata.

cord.

From the point of view under which we have thus presented it, the action of the spinal cord is therefore simple, or it is disturb- General funced by the agency of the brain; in the first case it offers it- tions of the self purely as an automatic instrument; in the latter, its commissural connections with the brain make a compound apparatus. The former state is closely represented in the construction of the amphioxus, the nervous system of which has no rudiment of a cerebrum or cerebellum; in this animal, therefore, since also the sensory ganglia are merely in a rudimentary state, the mode of life must be purely mechanical, just as it is with an artificial automaton, of which, when a given spring is touched, a given motion is made. Even among the highest vertebrated animals, man himself at the periodic times of quiescence of the cerebrum, as in sleep, when the cerebral influence over other portions is, to a certain extent, suspended, an approach to a similar condition occurs; but in periods of activity of the cerebrum, it can hold the spinal cord in check,