Though the medullary system is only met with in the bones, and though its principal uses appear to relate entirely to them, yet its properties and life differ so much from the properties and life of these organs, that we are compelled to examine them in a separate manner.
We distinguish two kinds of medullary systems; one occupies the texture of the cells of the extremities of the long bones, and the whole of the interior of the short and flat bones; the other is found only in the middle part of the first; let us examine each separately.
ARTICLE FIRST.
MEDULLARY SYSTEM OF THE FLAT AND SHORT BONES, AND THE EXTREMITIES OF THE LONG ONES.
I. Origin and Conformation.
This system appears to be the expansion of the vessels which penetrate the bones through the foramina of the second order, that is to say, through those that go to the common texture of the cells. These vessels having arrived on the internal surface of the cells, divide ad infinitum and anastomose in a thousand ways. Their interlacing gives to the interior of the texture of the cells that red appearance that characterises it, and which is so much the more evident, as it is examined at an age nearer infancy, because in fact the vascular system which is very evident at this period, becomes contracted and effaced as we recede from it.
These are the vessels which, in the section of the bones of the cranium by the trephine, give to the saw-dust the redness that is observed. It is these that produce the same phenomenon in the amputation of the extremity of the limbs. Though in general they remain loaded with blood at the moment of death, yet we can, as I have often done, accumulate in them still more by fine injections, which drive before them that which is found in the vessels, and concentrate it at their extremities; then the spongy texture of the adult is almost as red as that of the child which has not been prepared.
II. Organization.
Authors speak of a delicate membrane that lines the interior of all the osseous cells, and which they consider to be the exhalant organ of the medullary fluid. I have never been able, though my researches have been numerous, to discover a similar membrane. We see only the vascular elongations of which I have spoken, which, greatly multiplied, appear in fact to form a membrane, but when examined attentively are found to be very distinct from each other, not continuous, except at the place of the anastomoses, and leaving between them many small spaces in which the bone is not covered, but is in contact with the medullary fluid.
The exhalation then of this fluid appears to arise only from this vascular net-work, and in this respect it is analogous to that of the compact substance, which evidently contains no membrane, and the pores of which are however found filled with this medullary fluid, as is proved by the combustion of the compact texture and its exposure to the sun or artificial heat.
III. Properties.
This vascular net-work has only organic sensibility and insensible organic contractility, which are necessary for its functions; and it is this which especially distinguishes it from the medullary system of the middle part of the long bones, whose animal sensibility is, as we shall see, very great. Irritate in a living animal the interior of a short or flat bone, or the extremity of a long one, no sign of animal sensibility is manifested. Sawing the cranium, the condyles of the femur and the head of the humerus is not painful.
Injuries of this system when they are very great may produce necrosis of the bone, and the formation of a new osseous substance at the expense of the periosteum; but if a small portion only is affected, this phenomenon does not take place. I have many times perforated transversely with a gimblet the extremity of a long bone of an animal, and afterwards passed a red hot iron through the opening; the animal has always recovered without necrosis; the articulation has only remained swelled, and much injured in its motions, and some scales have come from it during the suppuration.
IV. Development.
The vascular net-work which forms this medullary system, exists in the cartilaginous state; but then, on the one hand, it does not admit the red portion of the blood, and on the other, the interstices of its meshes are found so filled with gelatine, that the cartilage appears homogeneous. At the period of ossification, the red blood penetrates on one side of these vessels, whilst on the other they become evident from the absorption of gelatine at the place of these cells, upon the internal surface of which they ramify.
In the foetus and the first age, this medullary system has a remarkable arrangement. It contains scarcely any of this oily fluid, from which it borrows its name, and which afterwards fills in so great a proportion the interstices of the texture of the cells of the different bones; by examining these organs comparatively in the different ages, I easily convinced myself of this. 1st. Exposed to a considerable degree of heat, the texture of the cells of the bones of an adult has an abundance of oily fluid flow from them. From the same experiment in the foetus, there only follows a drying of this texture by the evaporation of the fluids which enter it. 2d. If we burn the extremity of a long bone of an adult, the combustion is spontaneously supported by the oily fluid that escapes from the pores of the second species, and which keeps up the flame until it is exhausted. In the foetus, the bone ceases to burn when we take it from the fire, because the fluids it contains do not support combustion. 3d. Nothing is more difficult than to keep the bones of the adult white, because the oil that is in their interstices always yellows them a little. In the foetus and the infant, in whom this cause does not exist, the bones are easily kept white. 4th. By ebullition, we extract scarcely any oil from the texture of the cells in the first age; much swims on the water in which we have boiled this texture in the following ages. In general, the, foetus appears to want this oil entirely; it is formed after its birth, and its proportion is constantly increasing until complete growth. What fluid supplies its place in the first years? At first a large quantity of blood; for in general the redness of the medullary system is in the inverse ratio of the oil that is found in it; but the interstices of the cells appear moreover to be moistened by a fluid with which we are unacquainted, and which evaporates, as I have said, when we expose to the fire the bones of a foetus.
ARTICLE SECOND.
MEDULLARY SYSTEM OF THE MIDDLE OF THE LONG BONES.
This system differs essentially from the preceding in its nature, its properties, its functions, &c. It occupies the centre of the long bones, whose great cavity it fills.
I. Conformation.
Each of the organs from the whole of which it results, exhibits it under the form of a delicate membrane, lining the whole cavity, folded a great number of times, giving origin to many elongations, of which some cover the fine threads of the texture of the cells which are met with in this cavity, others pass, without adhering to any osseous portion, from one side of the membrane to the other, and of which all form numerous cells in which the marrow is contained.
We can then form of this organ an idea analogous to that which the cellular organ gives us; viz. that of a spongy body with communicating cells. The place that it occupies, gives to it as a whole, a cylindrical form.
It appears that at the two extremities of the canal, the cells or membranes do not open into those of the texture of the cells, and that the medullary fluid of the preceding system does not communicate with the marrow of this. In fact, the line of demarcation which separates them is evident; they do not mix in a gradual manner. Air injected from one side of the medullary cylinder, only penetrates with difficulty and by tearing the membrane, into the texture of the cells of the opposite extremity of the bone; yet, notwithstanding these considerations, I confess that the question is not fully settled. The transudations in dead bodies have no influence in deciding it, on account of the permeability that our parts acquire after death.
II. Organization.
The texture of the medullary membrane is very little known, because its extreme tenuity conceals it from our researches; for it is only in the bones of ricketty subjects, that its morbid increase in thickness has permitted me to trace it accurately. It has the appearance of cellular texture; yet its properties and its nature are very different from this texture; it cannot be referred to any of the three classes of membranes, the serous, the fibrous or the mucous. Some have pretended that it was an expansion of the periosteum, which had passed through the numerous foramina by which the bone is perforated, and entered into the medullary cavity; but the least parallel made between these membranes is sufficient to make us see that they are essentially different in their functions, vital forces, &c. and cannot have the same texture. A principal vessel penetrates the medullary membrane; it is the artery, which enters by the only, but very large foramen, which is seen on the body of the long bones; the two branches of this artery and those of the corresponding vein, ramify in an opposite direction in the medullary cylinder, and by their innumerable branches give to it a very evident reddish colour, that disappears with age. The extremities borrow their vessels from those of the neighbouring texture of the cells. We cannot trace any nerve there. Such is sometimes the abundance of the fluids which penetrate this membrane, and its extreme tenuity, that it might be said not to exist. To be convinced of its existence, expose the cylinder that it forms to the intense action of heat; it contracts, has the horny hardening immediately like all the solids, and thus becomes more apparent.
III. Properties.
The properties of texture are very well marked in the medullary organ. 1st. The spina ventosa in which this organ is distended in a very evident manner with the body of the bone, proves its extensibility. 2d. The contractility of texture is made apparent by the contraction of the cells, after the amputation of the middle part of a long bone, a contraction which prevents the flow of marrow, which without it would take place on account of the communication of these cells.
It is probable that the insensible organic contractility, which is then brought into action by the contact of the air upon this membrane which contracts from its irritation, has an influence also upon this phenomenon; for this membrane evidently has this kind of contractility, as well as the corresponding sensibility.
The animal sensibility is developed in it to an extreme degree in the natural state; the most acute pains are the result of the action of the saw upon it in amputation, of the introduction of a probe, of the injection of an irritating fluid into the medullary cavity, or of any other means which powerfully excite it. I do not speak of the pains of the bones in spina ventosa, syphilis, &c.; as the membrane is not then in a natural state, we cannot infer from them what kind of vital forces it is naturally endowed with. I have observed that the sensibility is greater, as we approach the centre of the bone with the probe when pushed into living animals. At the extremity of the medullary canal this sensibility is small; in the middle, the division of the bone is very painful. Whence arises this inequality of sensitive power, this decrease from the centre to the extremities? I know not. The medullary system evidently does not possess animal contractility and sensible organic contractility.
It is evident from this view of the vital forces which animate this system, that the life is much more active in it than in the osseous system, that its vital phenomena are consequently more rapid, that they have not that chronic course which characterizes all the diseases of the bones and that they respond more promptly to the sympathetic excitements of other organs. I am persuaded that many of the uncertain pains which we usually refer to the bones in diseases, have their seat rather in the medullary system, in that of the middle of the long bones especially; observe in fact that most of these pains are fixed in the middle of the limbs, and that they are really in the direction of that system. The medullary system of the extremities of the long bones, and of the flat and short ones, certainly enjoys much more vital energy than the osseous texture itself; inflammation is much more easily developed in it, its effects are more promptly shown. Who does not know that caries is so much the more rapid, in proportion to the quantity of the texture of the cells that exists in the bones? It is not this texture, which by its nature, has an influence upon this phenomenon; but it is, because the more abundant it is, the more the medullary system predominates in it; now as this participates in all its affections, it imprints upon them a rapidity which they have not in the compact texture in which it does not exist.
IV. Development.
This membrane exists in the cartilaginous state of the middle part of the long bones; but then it serves for the nutritive parenchyma to the gelatine that is exhaled there, and which, accumulated in very great quantity in its cells, renders the bone homogeneous in appearance, and prevents it from being distinguished. When ossification takes place, this substance is absorbed; the medullary cavity is formed; the medullary membrane is bare; the blood enters its vessels, till then permeable only by white fluids, because its kind of organic sensibility changes. Instead of receiving gelatine in its cells, it is the marrow or another fluid that it admits there, a phenomenon also dependant upon this change of organic sensibility. Hence an external form wholly new, a new organ in appearance, whilst in reality it is not the organ which changes, but the fluid that is deposited in it. The same phenomenon nearly is observed in the formation of callus, in which the portion of the medullary membrane corresponding to the fracture is at first cartilaginous, then osseous, and finally becomes what it was originally.
The exhalation of the marrow does not commence when the blood enters the medullary canal, or rather it commences, but I have found that it is wholly different from what it is afterwards in the adult. The proportion of oily substance is almost nothing in it, compared to what we have seen in the medullary fluid. 1st. It has a mucilaginous and reddish appearance; pressed between the fingers, it does not give out an oil as in the adult, but a fluid like gelatine. 2d. By comparing the water in which the marrow of the two ages has been boiled, we cannot see in the first, as in the other, many oily drops floating on the surface. 3d. Exposed to the action of fire, the middle of a long bone lets fall an infinite number of small burning drops, very beautiful, of a blue tinge and which are furnished by the marrow, which burns after being melted. Nothing similar to this is observed in the foetus. 4th. We know that the taste of the marrow is very different in young animals, in veal, for example, from what it is in adult ones. It is insipid, disagreeable, little esteemed in the first. 5th. I have observed that the marrow of children soon putrifies, becomes green, then black, if their fresh bones have been kept for some time in the air. The odour of this putrid marrow is very fetid. Preserve, on the contrary, for some time the bones of an adult, you will observe that their marrow turns rancid, and becomes of a deep yellow colour, like all fat that has been some time kept. In general the action of the air is wholly different upon the medullary organ, in the first and in the after ages. What is the fluid which this organ especially separates in the foetus and in childhood, and which then takes the place of the oily substance? It is an interesting object of research. Do the vulgar, who connect the idea of fat with that of marrow, know this phenomenon, when they say that children have yet no marrow in the bones? This absence of medullary fat in the foetus, essentially distinguishes the marrow from the ordinary fat, which, at this age, is already very abundant.
Functions.
The first and principal use of the medullary organ is to separate the marrow from the mass of blood by means of exhalation, for it has no glands, and afterwards carry it into it again by absorption, when it has remained for a certain time in its reservoir. This double phenomenon is very analogous to that which takes place in regard to the fat, for which we see that there are two orders of vessels distinct from the sanguineous, that enter its texture; it is not possible however to demonstrate them anatomically.
Is the activity of the exhalants varied by exercise or rest, heat or cold, corpulency or emaciation? We have not any precise experiment upon this subject, though numerous conjectures have been made. But we know that in phthisis, dropsy and marasmus, and in general, in all those states of the body in which general debility is carried to an extreme by the slow and gradual loss of the forces, the marrow, like the other fluids as well as the solids, is changed, loses its essential characters, its consistence and takes an appearance wholly different, without however the medullary membrane experiencing any lesion, or being thickened. I have never observed this lesion except in rickets. The appearance of the marrow in these diseases is mucilaginous, gelatinous, similar to what is seen in the foetus, with the difference of the redness, which is produced in the first age, by the great number of blood vessels.
The medullary membrane has a direct relation with the nutrition of the bone, a relation which has been proved by the beautiful experiments of Trojat, from which it follows that the destruction of this membrane produces the death of the bone, which has necrosis and is replaced by new bone, for which the periosteum serves for the nutritive parenchyma. These experiments are usually made by sawing a long bone at its extremity, and introducing into the medullary cavity a red hot probe, which destroys the whole organization. Soon after the periosteum swells, inflames and has an extreme sensibility to the external touch. This sensibility is gradually lessened; the inflammation disappears. A considerable quantity of gelatine penetrates the internal layers of this membrane, which becomes a cartilaginous sac, with which the bone is covered. At the end of some time, which varies according to the class of animals subjected to the experiment, according to their age, their temperament and other causes, the vascular system, destroyed on the interior of the canal, and expended wholly upon the periosteum, deposits there the phosphate of lime destined for the bone. To the cartilaginous cylinder then succeeds the osseous one. The bone within has no connexion with the life of the living body that surrounds it on all sides. There are then in artificial ossifications three very distinct periods, 1st, swelling and inflammation of the periosteum; 2d, cartilaginous state of the internal layers of this membrane; 3d, osseous state. Besides, these two last states are not as regular and distinct, nor as easy to be observed as in natural ossification.
Does the medullary membrane serve indirectly to furnish a part of the synovia by the transudation of the marrow through the extremity of the long bones? Most authors assert it. We know at the present day, what must be thought of these mechanical transudations, which are observed in dead bodies, but which are repugnant to the known phenomena of vitality; besides, the following experiment leaves no doubt upon this point. I have opened upon the sides two long bones of one of the hind legs of a dog, so as to pass in a red hot probe, which having been carried in several times, completely destroyed the two medullary systems; necrosis has been pretty soon the consequence of this experiment, and yet the articulation between the two bones with necrosis, has continued to receive synovia as usual, a circumstance that would not have happened, if the transudation of the marrow was necessary to the production of this fluid. Who does not know, on the other hand, that in diseases of the articulations in which the synovia is altered and vitiated, the marrow of the corresponding bones is almost always in a perfectly sound state, and that vice versa, in the diseases which attack the medullary organ, the synovia is not altered in its nature like the fluid which this organ contains in its cells?
