Part 35

OF THE FIBROUS MEMBRANES IN GENERAL.

After having considered the fibrous system in a general manner, as it relates to its organization, its life, its properties and its nutrition, I shall now examine it more particularly in the great divisions it offers, and which we have pointed out above. I begin with the fibrous membranes.

I. Forms of the Fibrous Membranes.

These membranes which comprehend, as has been said, the periosteum, the dura-mater, the sclerotica, the albuginea, the peculiar membrane of the spleen, the kidneys, the corpus cavernosum, &c. are almost all destined to form external coverings, kind of sacs in which are contained the organs they invest.

These organs are not, like those around which the serous surfaces are spread, as the stomach, the intestines, the bladder and the lungs, subject to alternate dilatations and contractions. This would not accord with their degree of extensibility. They are fitted exactly to the form of these organs, and have none of those numerous folds which we see in the serous membranes, if we except however the dura-mater. Their two surfaces are adherent; a character which distinguishes them especially from the preceding membranes, as well as from the mucous.

One of these surfaces, intimately united to the organ, appears to send various elongations into it, which identify at first view its existence with that of the membrane. Many fibres detached from the albuginea, from the covering of the corpus cavernosum, from the peculiar tunic of the spleen, &c. or rather adhering to these tunics, penetrate the respective organs of these membranes, and crossing there in various directions, form as it were the outline and frame, around which are arranged and supported the other constituent parts of these organs, which seem from this to have the external membranes for a mould; as we see when these moulds are removed, irregular vegetations shooting up here and there. The callus, in displacements too great to allow the periosteum to extend over the divided surfaces, is rough and uneven. The form of the testicle alters, when the albuginea has been divided at any part. This adherence of the fibrous membrane which covers different organs, to the internal elongations of these organs, and the fibres which compose their outline, has made anatomists believe that the nature of one was the same as that of the others, that these were but elongations of the membrane. I thought so when I published my Treatise on the Membranes; but new experiments have since convinced me of the contrary.

I am confident that the membrane of the corpora cavernosa belongs to the fibrous system alone. The internal spongy texture, contained in the cavity of this membrane, has not the nature of it, is not as all anatomists say an elongation of it. The spongy texture is not made by laminæ, which, according to the common expression, are detached from the membrane and produce it by their interlacing. This is a separate body, unlike in its life and its properties.

By exposing a corpus cavernosum to ebullition, I have evidently observed this difference; the external membrane does, like all the fibrous organs, become thick, yellowish, semi-transparent, then melts more or less into gelatine; the spongy texture, on the contrary, remains white, soft, does not increase in size, hardly crisps at all from the action of fire, exhibits, in a word, an appearance which I can compare to no texture treated in the same way by ebullition.

Maceration also answers very well to distinguish these two textures. The first yields but slowly to it; its fibres remain a long time distinct; they have still their natural arrangement, when the second is already reduced to a homogeneous, reddish pulp, in which nothing fibrous, nothing organized can be any longer traced. In general, it appears that the spongy texture of the corpora cavernosa is their essential part, that in which the great phenomena of erection take place, that which animates the peculiar kind of mobility which distinguishes it from the other organs. The fibrous shell is only accessory to its functions; it is but a covering; it is only formed to obey in erection the impulse which is communicated to it.

When we expose the corpus cavernosum to the action of the nitric acid, the spongy texture, freed from the blood it contained, becomes of a much deeper yellow than the fibrous membrane; this enables us to distinguish them clearly from each other.

By exposing the testicle to the action of boiling water, we also observe that its internal texture assumes an aspect wholly different from that of its external membrane; it becomes of a deep brown, whilst the other remains white; it does not assume the gelatinous appearance in so decided or in so prompt a manner as that of the corpus cavernosum.

Subjected to maceration, the testicle is also wholly different in its covering and in its internal texture.

The surface of the fibrous membranes, opposite to that which corresponds to their organ, is joined to the neighbouring parts, sometimes in a loose manner, as the covering of the corpus cavernosum, sometimes by very tight bands, as the dura-mater. In general the membranes, and even all the fibrous organs, have a singular tendency to unite intimately to the serous and mucous surfaces. We find examples of this in the serous membranes in the union of the dura-mater with the arachnoides, of the albuginea with the tunica vaginalis, and the fibrous capsules with the synovial. Such is the intimacy of this adhesion, that the most careful dissection cannot destroy it in adult age. In infancy, it is much less, as we see very well especially in the relation that exists between the base of the pericardium and the phrenic centre, a relation which is such, that we can with ease separate in the first age the two surfaces which are rather contiguous than continuous, whilst in the after ages we are unable to do it.

As to the union of the mucous surfaces with the fibrous, when they are found contiguous, they are entirely confounded; this is observed in the pituitary membrane, in that of the sinuses, of the ear, &c. The perichondrium of the larynx and of the trachea is only a part of their internal membrane. In all these parts, the periosteum so intermixes with the mucous surface, that it is impossible to separate them, and they are removed together from the bone, which then remains bare. The vas deferens, the fallopian tubes, the ureters, &c. are also very evidently fibro-mucous.

II. Organization of the Fibrous Membranes.

The fibrous membranes have in general a very compact texture, of a remarkable thickness; they are formed only of a single lamina. The dura-mater seems to be an exception to this rule, as its folds form the falciform process and the tentorium cerebelli; but except at the place of the sinuses, it is difficult and even impossible, to find two distinct laminæ.

These membranes have more vessels than all the other divisions of the fibrous system; they are perforated by a great number of foramina for the passage of these vessels, most of which only pass through them, and afterwards go to the organs they cover. These foramina, each of which is larger than the branch it transmits, form also a character of the fibrous membranes, distinct from the serous, which are always folded up, and never open to allow the vascular system to penetrate their respective organs.

The particular description however of the membranes of which we are treating, will be added to that of the organs they surround. I shall except the periosteum, whose description belongs to and may be made in a general way, whether because clothing the whole osseous system, we cannot consider it separately, or because, as I have said, it is the centre from which arise and to which go all the organs of the fibrous system, so that its functions relate still more to this system than to that of the bones.

III. Of the Periosteum. Of its Form.

This membrane surrounds all the bones. Hard, resisting, of a grey colour, it forms for them a covering which extends everywhere, except where the cartilages cover them. Its thickness is remarkable in infancy; it is thinner in proportion in the adult and becomes more firm and compact.

The ancients described it as extending from one bone to another over the articulation, and thus forming a continuous sac for the whole skeleton. This idea is incorrect. At the junction of the bones, the periosteum intermixes with the ligaments which serve it as a means of communication, and it is in this way only that we can understand its continuity. The crown of the teeth is destitute of it, as well as all the osseous productions that grow upon the head of certain animals.

The periosteum is feebly united to the bone in infancy; it can then be separated with great ease, especially on the middle part of the long bones. In the adult, as the calcareous substance gradually encrusts its most internal fibres, the adhesion becomes very strong; it is extreme in old age, in which this membrane is often reduced to a very delicate layer by the progress of ossification. The constant pressure exerted by the muscles in their contractions, can also have a little influence upon this adhesion. Various elongations pass from the periosteum to the bone. They are much more numerous at the extremities of the long bones and upon the short bones, than upon the middle of the long bones, or the broad ones; which may easily be conceived of, from the much greater number of foramina in one than the other part. These elongations accompany the vessels, line the canals which pierce through the bone, are lost in those which terminate in its substance, do not penetrate the medullary cavity, and confined to the osseous texture alone, establish, between it and the membrane from which they arise, immediate relations.

It is the destruction of these relations, when the periosteum is diseased or destroyed for a considerable extent, that produces the death and separation of the bone. There is however this difference between this phenomenon and the death of the bone by the injury of the medullary membrane, that if this is disorganized, necrosis seizes upon the whole bone, whilst if we irritate and tear the periosteum in the middle part of a long bone, for an extent nearly equal to that of this medullary membrane, the external laminæ of the compact texture alone are detached by exfoliation, and the bone remains the same. I have made this experiment the year past upon two dogs. As to that which consists in removing the periosteum, not only from the middle part, but from the whole surface of the bone, I do not know that any one has been able to try it; it has appeared to me impossible; it might be practicable, but the animal would soon die from the extent of the injury, and thus we could have no result from it.

The relations of the periosteum with the neighbouring organs vary remarkably. In the greatest number of bones, there are muscles that slide upon it; the cellular texture unites it to them more or less closely according as the motions are more or less considerable. In consequence of inflammations, it loses this laxity, and often all motion ceases.

Organization of the Periosteum.

The direction of the fibres of the periosteum is nearly analogous to that of the bones, the long bones especially as well as the short; but it has not the radiated structure of the flat bones that it covers. These fibres placed upon each other, have different lengths; the superficial ones are the longest; those which correspond immediately with the bone run but a short distance. In general all become very evident in some diseases of the bones. I recollect among other examples of the preternatural development of the fibres, a man affected with elephantiasis, and at the same time a swelling of the compact texture of the tibia, which was of a remarkable thickness. The periosteum of this bone was very thick, and adhered so little to the bone, that the slightest effort was sufficient to raise it in its whole extent, and its fibres were so distinct, that it might have been taken, when separated from the bone, for a portion of plantar or palmar aponeurosis.

The periosteum borrows its vessels from those of the neighbouring parts. Their innumerable branches ramify in it ad infinitum, form there a net-work, which injections, especially in infants, make very evident, they are afterwards lost in it, or penetrate the compact texture of the bone, or return to the neighbouring parts to form anastomoses.

This membrane receives, as we have said, the insertion of almost all the fibrous system, of the tendons, the ligaments and especially the aponeuroses. This insertion has no connexion with the bone in infancy; but ossification soon seizing upon the most internal laminæ, all the fibrous organs appear to be identified with the bone in the adult. I would observe that this arrangement coincides with the prodigious power of drawing that the muscles, having become more developed, often exert at this age, and which only spent upon the periosteum, as it would have been without its ossification, would not have found in it a sufficient resistance, whereas acting also upon the bone, it moves it without endangering its covering. The general organization, the properties and the life of the periosteum are the same as those of the fibrous system; I shall not treat of them.