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done but little of this kind of work. They should do more. The field is a large one, and promises excellent results.
(To be continued.)
NEKVOUS MATTEE AND PEINOIPLES OP NEEVOUS AOTIOF.
BY GARRETT NEWKIRK, M.D., CHICAGO, ILL.
(Read before tbe Illinois State Dental Society, May 13, 1885.)
In the material world around us the forces are the physical and the chemical. In the vegetable there is superadded the mysterious force we denominate vital. In animal life alone do we find the nervous.
The distinctive feature of the animal kingdom, from the lowest to the highest forms, from the simplest to the most complex, is the possession of nervous matter, either in visible structure or as a property of protoplasmic fluid.
In man we have the same physical, chemical, and vegetative forces that exist in external nature, but in a sense these are all subordinated to the energies resident in his mass of nervous tissue.
The first step in the division of nervous tissue is into the gray and the white. These two are intimately associated and correlated. They lie side by side in brain, and cord, and ganglion. There is no abrupt line dividing them. They are, as it were, interlaced. The gray tint is not suddenly but gradually effaced in the meshes of the white. Of the two, the gray matter is the more homogeneous—more distinctively cellular in structure. The white tends to parallelism of arrangement more and more as it recedes from the gray till it completes its purpose in the formation and distribution of nerves. The white is the firmer of the two, not so much by inherent difference of constitution, but rather by the addition of interstitial layers of fibrous connective-tissue. Consequently we find the gray matter so placed as to receive the best possible protection from external causes of injury. In the cord it is inclosed and protected by the white, and both are guarded from injury by a most ingenious arrangement of the bones that form the spinal canal—an arrangement which permits a certain amount of motion without danger to the great nerve trunk. Within the skull this order is reversed. The gray matter is on the outside—spread out, so to speak, on the surfaces of the convolutions, and lies just within the cranial walls. But it is well protected from shocks.
First, the skull is double-plated, the two plates being in places somewhat separated by spongy bone.
Secondly, three distinct membranes lie between the skull and the gray matter, forming together a protective cushion.
Again, the arrangement of the folds or convolutions is such as to permit a degree of lateral movement of the folds themselves, valuable as against the shock of external violence.
This arrangement of the gray matter on the surface and in folds allows the placing of the greatest amount with the least thickness at any given point; and if an organism so jelly-like were otherwise arranged, it would be in danger of breaking down by its own weight, or the weight of other matter, or from jars, shocks, or congestion.
We may say of the gray matter that it is the center of animal life and action. So far as we know, there is no form of life where the white exists without the gray, or some form of protoplasm which represents it. And the white is servant to the gray,—subordinate both in motion and sensation.
Eegarding the structure of gray matter, it is sufficient for our purpose here to say that it is little more than an aggregation of nerve-cells, with just sufficient connective nerve-tissue to hold it in form.
But as we know the constitution of the ocean by the analysis of a single drop of water, so we may understand quite well the character of nervous matter in mass when we have learned the structure and habits of a single cell, or a series of connected cells.
We have in Fig. A a simple sensitive nerve-cell, with a motor appendage, as found in some of the lower forms of animal life. In Fig. B a higher form is represented, where the sensitive cell is connected by a nerve to a muscular cell. Fig. C illustrates another advance in organization; a sensitive cell connected by a sensitive nerve to a central cell, which in turn communicates by a motor nerve with the muscular cell.
This last is the triple alliance—the triangular base of the higher order of nerve-structure. Given an understanding of this simple series, and you have the underlying principles of all nervous action. Multiply first the sensitive cells into millions of such cells. Let these be distributed through all the tissues of the body, but chiefly on the surfaces, internal and external. Bach cell is to take cognisance of whatever comes in contact with itself. It is an intelligence officer to the central cell.
By the term sensitive we mean all cells which receive impressions from without and transmit to the center, and this whether the mind be conscious of the impression or not—whether the center be the brain or cord, or a simple ganglion.
The sensitive cells perform a vast amount of work dis-sociated from mental consciousness. The mind is not troubled by the ordinary and healthy processes of organic life. These are under the control and supervision of the lower ganglia, with which the sensitive cells of the heart, lungs, and other vital organs communicate. In fact, the sensitive cells take cognizance of many things of which the mind cannot be cognizant—insensible properties, such as exist in deadly poisons. The sensitive cells are the sentinels on guard at every outpost; they preserve every interest in every tissue. By this intelligence nutrition is regulated and supply made equal to the demands of waste; upon their information glands secrete in due amount, and the circulation is hastened or retarded.
But we must let our minds be plainly impressed with one distinctive fact as to the sensitive cell. It does not originate anything, so to speak. It is simply an informer. It is not a governing cell. It is a news reporter, but not an editor. Its only connection with the central office is by wire, and its function is to send messages but never to receive any.
Each sensitive cell has its own wire; so we assume, although it cannot be demonstrated but that a small group may use the same line—namely, a filament, a nerve fibrilla, a part of the so-called sensitive nerve. This wire transmits but one way. The sending office is always at the same end, the peripheral; the receiving is the central cell. The central is a cell of gray matter either in ganglion, cord, or brain. The gray matter would seem to be the seat of independent power. It sets in motion. It develops energy. It has a will of its own. It sits as a ruler and a governor.
Now, the whole mass of gray matter of the great cerebrospinal ganglion is simply an aggregation—a myriad multiplication of the
We have seen that the sensitive cell has but one office and relation to tne system, viz., to transmit messages from periphery to center.
In its simplest office the central cell has at least a double relation. First, it receives the impulse sent by the sensitive cell. Second, an amount of energy is at once developed according to the impulse received, and manifested without hesitation in the direction indicated.
A finger of the hand is pricked by a needle. The central cells are almost simultaneously informed; a sufficient energy is induced to withdraw the hand. The time taken for it all is quite infinitesimal, but there is no mistake made between hands or a hand and foot. The nerve fibrilla by which the message goes to the muscle to withdraw the hand differs in no wise, so far as we can see, from the fibrilla which carried the message of information. Tbey seem to be but simple wires. But they do not lie in contact. A bundle of one kind is laid together inclosed by a common sheath, and when we see it we call it a sensitive nerve; a bundle of the other kind we say is a motor nerve, because we have learned by experiments what offices are performed by each.
So far as we know, the peripheral or sensitive cells do not associate so as to pass impressions from one to another, although a great multitude may be simultaneously affected by the same cause. Thickly distributed as they are, they are distinct. They are distributed in and upon other tissues.
But the central cells exist in mass. The gray matter contains barely enough connective-tissue to form a skeleton and supply blood. The central cells, therefore, are not only intimately connected with the sensitive cells by the sensitive nerves, and contractile tissue by the motor nerves, but also with each other.
So the cell, receiving an impulse from the periphery, may associate with itself a thousand cells of equal power, and, multiplying the force a thousand times, send it on the instant by as many motor fibrils to produce contraction.
The change in the muscle cells is a measure only of the change in the central cells.
Common substitutes for the words sensitive and motor are the terms afferent and efferent, meaning to convey in and out. So the central cells are said to convert afferent into efferent impulses, or sensation into motion.
Having seen how a slight afferent impulse may be multipled by the central ceils into a manifestation of force greatly disproportionate (as, for example, in a case of convulsion from slight peripheral irritation), it is evident there must be some means of limiting such extreme action; otherwise there would be continually, as there is occasionally, in abnormal conditions, great waste of energy.
So there are, first, limitations of anatomy, so to speak. The cord and brain are in segments or divisions, each division being set over special parts and functions. The dividing lines may not be traced, but there is no doubt of the fact of their existence. Therefore, an afferent impulse sooner or later reaches a point beyond which it may not readily go.
Secondly, there are certain tracts, so it is believed, the office of which is inhibitory. That is, it is their business to "put on brakes." We know there are such tracts of matter in the brain, because, by stimulating certain nerves that proceed from these parts, we can slow the action of the heart and lungs. Medicines like veratrum viride and digitalis act upon certain central cells, and through them by fibers of the pneumogastric and spinal-accessory nerves. Stimulation of these fibers inhibits heart action,—reduces the number pf beats per minute.
When we speak of a nerve, the idea in our mind is ordinarily that of unity. We think of the nerve as one thing. But let us examine for a moment one of these, say a spinal nerve.
A spinal nerve consists of two nerves. These have entirely different offices. They go together merely for the sake of convenience,— first, in getting out of the bony canal in which the cord lies, and, second, convenience of distribution. But we call the two together a nerve. Let us say, for illustration, it is a rope of two strands. These strands are made of threads, and the threads are bundles of fibres or filaments. We can see the rope, the two strands before they join, or we can separate the coarser threads so that they may be seen with the naked eye, but the filaments only with the microscope. Unlike the strands and threads of a rope, however, the parts of the nerve are not twisted upon each other, but lie parallel, inclosed by a common sheath.
The diameter of the fibrilla varies from the y^ to y^^- of an inch. Supposing the average to be ^Vfr* an(* multiplying this into itself, we have 25,000,000 as the possible number in a cord containing the equivalent of a square inch.
If Chicago were connected with the outside world by a like number of telegraph wires, these wires, laid as closely as possible, would fill four of the principal streets fifty feet deep. Allowing forty wires to each person, it would require the services of every man, woman, and child in the city to operate them.
But the aggregate of the diameters of the nervous trunks of the body would be not one but several inches.
The two strands of the rope—the spinal nerve—are to all appearances and analysis alike, yet they convey energy only in opposite directions. One (the posterior) is the afferent wire, conveying impressions from without inward. The other is the efferent wire, conveying motive force from the central cells to parts without.