General Anatomy, Applied to Physiology and Medicine, Vol. 2 (of 3)

by Xavier Bichat

Compact Texture.

The fibres that form the compact texture are not the same as those of the preceding. These fibres, being in juxta-position, not leaving any space between them, giving by their approximation a remarkable density to the texture they form, have a longitudinal direction in the long bones, are in the form of rays in the flat ones, and cross each other in all directions in the short ones. This triple arrangement of the fibres of the compact texture appears to be wholly owing to the manner of ossification. In fact, when we examine its progress in the primitive cartilages, we see these organs encrusted with the phosphate of lime, in the same direction which these fibres afterwards take. Thus these fibres are very evident in the first age, on the bones of the cranium in particular. When the phosphate of lime, successively deposited on the cartilaginous parenchyma, predominates there, then the whole is confounded in the compact texture in one homogeneous mass. But still there are different circumstances that indicate the primitive direction of the fibres: 1st. When by an acid we remove from bones their calcareous part, then the cartilaginous portion keeps as a kind of mould, the form of the substances that filled it, and exhibits fibres whose direction is the same as that pointed out for the three species of bones. If we wish to separate the cartilaginous layers, it is in this direction that it is most easily done. 2d. The fissures that come in bones long exposed to the air follow in general the natural direction of the fibres. 3d. Calcined bones exhibit nearly the same phenomenon.

The direction of the fibres of the compact texture is changed entirely in the apophyses, in which it does not follow that of the principal bone. In those, which by their form, partake of the character of the long bones, as in the styloid, these fibres are longitudinal; they go in all directions in those, which like the mastoid, the different species of condyles, &c. resemble in their shape the short bones.

The assemblage of the fibres forms, according to anatomists, layers which they have considered as in juxta-position, and held together by little pins according to some, and by the interlacing of fibres according to others. These osseous layers do not appear to me to exist in nature. All the fibres of the compact texture adhere to each other, cross and form a whole that we cannot conceive of in this manner, and which besides does not accord with the irregularity of the distribution of the vessels. Art separates here fibres layer by layer, as it is done in a muscle, in a ligament, &c.; but these layers are wholly factitious; to exhibit the bones as formed by an union of these layers is to give a very inaccurate idea of their structure. It is still more inaccurate to consider these layers as attached to each other by osseous pins, by attraction, or by a glutinous matter which serves as a glue. All these ideas, contrary to anatomical examination, suggested by a false application of the laws of the adhesion of inorganic bodies to the adhesion of organized fibres, now belong only to the history of physiological errors. There is a circumstance, it is said, that very evidently proves the lamellated structure of the bones, it is their exfoliation. It is true that often very distinct layers are separated from the living bone, but these layers are only the product of exfoliation itself. Then in fact the bone dies on the surface; the superficial vessels receive no more blood; this fluid is stopped under the portion deprived of life, the exhalation of the phosphate of lime ceases there, every kind of sanguineous, exhalant and absorbent vessel is destroyed; a slow inflammation, with suppuration, comes on, and fixes the line of demarcation; and as this line is often at the same place, all which is above it becomes an inorganic layer which gradually falls off, and preserves its osseous solidity, because the dead absorbents were not able to remove the phosphate of lime. Besides, nothing is more common than to see exfoliation take place not by layers, and the bone afterwards exhibit an unequal surface, the effect of the inequality of the thickness of the exfoliated portions. Finally, exfoliation often takes place in a direction opposite to that which the layers are thought to have; this is what we see in the separation of the extremity of the long bones, that have been exposed to the air or too much irritated after amputation, in the shedding of the horns of animals, &c. Let us consider the compact texture as an assemblage of condensed fibres, not separated by layers, which we can only consider as imaginary.

The fibres of the compact texture differ in their organic arrangement, from the muscular fibres in this, that frequent elongations unite them to each other, whereas the muscular have only the cellular organ, the vessels and the nerves as the means of union. Such is the intimate juxta-position of the fibres of the compact texture, that they leave between them only pores hardly sensible to the naked eye, but which become so however with a glass, and which the medullary juice and vessels fill. In the rickets this density of texture disappears, and we observe in the middle part of the long bones and under the layer of periosteum more thick than common, an osseous texture, easily bent in all directions, forming an infinity of cells and taking the place of the compact texture that ought to be there. It appears that this change of compact texture into that of cells is made less by the absorption of a part of the phosphate of lime, than by the extension of the osseous fibres which separate from each other, and leave between them spaces that did not before exist; this gives to the bodies of long rickety bones a very considerable thickness. I have many times made this remark.

Arrangement of the two Osseous Textures in the three kinds of Bones.

The osseous textures, considered in the different kinds of bones, are differently arranged. In general the compact forms the exterior, the covering of the bone, and that of the cells occupies the interior. The ossa spongiosa form an exception to this rule, the modifications of which we shall now examine.

1st. In the long bones, the compact texture has a very remarkable thickness in the centre, where it serves the triple purpose, first of protecting the medullary organ, of which it is the covering, then of giving solidity to the bone in this place, which more than the extremities, is exposed to great efforts in locomotion, falls, concussion, &c. and where the bone, traversed only by some very weak fibres of the cells, cannot borrow its resistance but from its external parietes; finally, of thus diminishing without danger the size of the bone in the middle part of the limb, the form of which, becomes by this means much more regular. So that as we go from the centre, we see in a long bone, sawed longitudinally, the compact texture diminish in thickness, and form at last at the extremities only a delicate layer analogous to that which covers the short bones. Thus the power of resistance of the long bones, at their extremity, is less in their compact shell, than in the great quantity of the texture of the cells deposited under it; it is this especially that prevents fractures; hence we see how the proportion of the compact texture and that of the cells being inverse in the two parts of the bone, the manner of their resistance is also inverse.

The texture of the cells differs a little when examined in the medullary canal and in the extremities. In the canal there are extremely delicate filaments, continued from larger fibres which fill above and below the extremities of the bone, and the compact portion which forms the osseous cylinder. Few and scattered at random in the middle of the canal, these filaments approximate each other, and form a kind of net-work, as they go from it; hence the name of reticular substance by which it is designated. But it is not a distinct texture, it is only a modification of that of the cells; a modification, which is especially characterized, 1st, by the delicacy of the fibres; 2d, by the uniform absence of those fine and short layers which frequently belong to this texture in other parts. Besides, the manifest use of this portion of the texture of the cells, too weak to contribute to the resistance of the bone, is evidently to serve as a support to the medullary system, and insertion for its membrane. At the extremities of the long bones, the fibres of the texture of the cells increase a little, approximate each other, are scattered in layers, and give to the bone by their union and number, a remarkable thickness and resistance, without however increasing the weight, which very much favours locomotion, considering that this weight placed at the extremity of the lever would have been very painful to raise.

2d. In the flat bones, the compact texture forms two external layers, the thickness of which is between that of the middle of the long bones, and that of the extremity of the same bones, or that of the short ones. Between these two layers is found the texture of the cells, similar in general to that of the extremity of the long bones, a little more lamellated however, thicker usually at the circumference, often almost wanting in the middle of the bone, where its two compact layers in juxta-position allow a light to be seen through it, when placed behind. In general wherever the broad bones are so thin, from the want of the texture of the cells, there are very strong muscles, which by their thick layers give solidity to the bone. We see examples of this in the iliac, sub-scapular, inferior-occipital fossæ, &c.