Part 17

We have seen on the contrary that the primitive systems are everywhere continuous, having no interruptions in them. The cellular, the arterial, the venous, the absorbent, the nervous are so arranged, that if it were possible to remove all the organs they enter, and leave them alone, they would form a complete whole, formed differently according to the different systems. The exhalants can also be considered as everywhere connected, as we have seen. Suppose on the contrary that the organs intermediate to the bones, the cartilages, the fibro-cartilages, &c. should be removed, all the parts of these systems would immediately be separated, and you would not have one continuous whole.

The order to be followed in the examination of these systems is of no importance; we shall place them in the following order, which will comprehend 1st, the osseous; 2d, medullary; 3d, cartilaginous; 4th, fibrous; 5th, fibro-cartilaginous; 6th, animal muscular; 7th, organic muscular; 8th, mucous; 9th, serous; 10th, glandular; 11th, cutaneous; 12th, epidermoid; 13th, and finally, the system of the hair.

Observe that nature is not confined to any methodical order, in distributing these systems in the different apparatus; that she has no regard to the great differences that she has established between the functions. Each can at the same time belong to the apparatus of functions that have no analogy. Thus the fibro-cartilaginous, which is found especially in the organs of locomotion, and consequently in animal life, enters also by the trachea into the respiratory apparatus; thus the mucous system, everywhere destined to the organs of internal life, belongs also to the external life in the conjunctiva, in the nasal fossæ, &c. to generation in the vesiculæ seminales, in the prostate, &c.; thus the glandular system pours by turns fluids upon the organs of the two lives as upon those of generation; thus the serous surfaces are spread upon parts whose functions have no resemblance upon the brain and the stomach, for example, upon the articular cartilages and the lungs, &c. Let us consider then the simple systems abstractedly, if I may so say; let us describe them in an insulated manner as materials distinct from each other, though united two by two, three by three, four by four, &c. to form the partial edifices of our apparatus, edifices from which results the general edifice of our organs. Each of these apparatus is destined to exercise a determinate function, and ought consequently to be classed as functions; it is in this manner also that we shall distribute them in the Descriptive Anatomy. But the simple systems, not tending to a common object, except as they are united in the apparatus, we cannot, when considering them separately, confine them to any classification borrowed from their destination.

OSSEOUS SYSTEM.

This system, remarkable among all the others by the hardness and resistance that characterize it, has from this double attribute a fitness to serve as a common base for all, upon which they rest, and around which they are suspended and fixed. The whole of the pieces that form it, are connected together for this use, by means of flexible and resisting bands, which with these pieces make a whole that is called a skeleton. The osseous whole, placed in the midst of many organs that it sustains, everywhere continuous in its different parts, has not however, like the primitive systems, continuity of peculiar life from one of its extremities to the other. The bands which connect these different pieces, very different from them in their nature and their properties, produce in them an insulation of vitality, which the different parts of the above systems do not exhibit, because in their continuity their nature is everywhere the same.

ARTICLE FIRST.

Of the Forms of the Osseous System.

Considered in relation to their forms, the bones are of three sorts, long, flat and short. One dimension predominates in the first, viz. length; two are in nearly equal proportions in the second, length and breadth; these two last dimensions, with thickness especially added, characterize the short bones. Let us examine each in a general manner.

I. Of the Long Bones.

The long bones belong in general to the apparatus of locomotion, in which they form a kind of levers that the muscles move in different directions. All are placed in the extremities, in which their whole forms a kind of central column, moveable in different directions. We see them successively diminishing in length and increasing in number, when examined from the superior to the inferior part, from the thigh or the humerus to the phalanges of the toes or the fingers. It follows from this double opposite arrangement, that the top of the limbs is characterized by the extent of its motions, and the bottom by the multiplicity, variety and narrow limits of these motions.

These bones have all an analogous conformation; thick and large at their extremities, they are more slender and usually rounded in the middle or body, as anatomists call it.

The size of the osseous extremities exhibits the double advantage, 1st, of presenting to the articulations large surfaces and consequently more causes of resistance to different displacements; 2d, of contributing to the regularity of the forms of the limb to which they belong. Observe in fact that the muscles and the bones are placed in an inverse direction in the extremities. The middle of the first, which is their largest part, corresponds to the middle of the second, which forms their small portion, whilst the extremities of these compensate by their size for the smallness of the tendons which terminate the others, and which are placed at the side of them. The increase of size of the extremities of the long bones is not sudden; it commences imperceptibly upon the body. We observe upon these different extremities eminences of articulation and of insertion.

The middle or the body has no eminence; prominent lines are seen there, always destined for aponeurotic insertions, and which, when they are very considerable take from the bone its cylindrical form, which it however preserves in the interior; thus the tibia is evidently triangular externally, though within its canal has the form of that of the femur. In general these lines of insertion, always separated by plain surfaces, are three in number upon each long bone, as we see on the humerus, the radius, the ulna, the tibia, fibula, &c. I know not the reason of this law of conformation. Another general observation is, that the body of almost all the long bones is twisted, so that the direction of its superior part is not the same as that of the inferior; by tracing from above downwards one of these lines of which I have just spoken, this may be seen; it is however more evident in the adult than in the fœtus. This change of direction has no uniformity in the course it pursues.

The internal forms of the long bones are very well seen by sawing them longitudinally. The texture of the cells fills them to the extremities; it is, as we shall see, more fine and less abundant in the middle, where the medullary canal exists.

This canal does not exist in the first month of the fœtus, nor as long as the bone is cartilaginous; the osseous state is the period of its formation. All the gelatine of the middle of the bone is then absorbed, exhalation brings no more there, except in the very delicate texture of the cells that this canal contains; this function, which is nothing in the centre, becomes more active on the circumference of the bone. This increase of activity of the external exhalants favours the formation of the compact texture, the development of which takes place precisely at the same time as that of the canal whose parietes it forms; so that at this period of ossification, exhalation and absorption appear to be in an inverse state in the two parts of the bone; one is very active on the exterior in bringing phosphate of lime, with which it encrusts the already existing parenchyma; the other is very active in the interior in removing the gelatine whose absence forms the space from which the medullary canal arises.

There is no well marked medullary cavity except in the humerus, the radius, the ulna, the femur, the tibia, the fibula and clavicle. The ribs and the phalanges, which in their forms resemble them, have much of the ordinary texture of the cells in their centre, and hardly ever any of that more delicate texture of the cells which occupies the centre of the bones above named, and which is only found in the medullary cavity.

This cavity does not extend beyond the body of the bone; where the compact texture grows thinner, it disappears, and is replaced by a great quantity of the texture of the cells, which fills the extremity of the bone. Its form is cylindrical and its direction straight. It does not vary in its form, on account of its asperities or the external prominent lines of the body of the bone, which is only thicker in these places. Its parietes are much smoother in the middle, than at the extremities, where there are already many considerable cellular filaments thrown off. There are in many subjects, delicate, horizontal bony partitions, which interrupt almost entirely its continuity in this place, and appear to divide it into two or three very distinct parts.

The medullary canal serves not only to lodge and defend the medullary organ, but also to give more resistance to the bone; for we know, that of two cylinders formed of an equal quantity of matter, one of which is hollow, and consequently has a greater diameter than the other which is full, the first will resist more than the second, because we can bend and break it with less facility. Full cylinders, equal in diameter to the long bones, would have prevented by their weight, the motions of the limbs; whilst other cylinders of the same weight as the present, but without any cavity, would give too small a surface for the insertion of the muscles. To unite small weight with a sufficient space in the middle of the long bones, is then a great advantage of the medullary canal.

This canal disappears in the first periods of the formation of callus in fractures, because the whole medullary organ is occupied at this place by gelatine, and becomes cartilaginous; then this gelatine gradually re-absorbed, without being replaced, favours the development of a new cavity, and the communication is re-established between the superior and inferior parts of the canal.

I have observed that, in the first age, and while the extremities of the bones are cartilaginous, the medullary canal is shorter in proportion than in the adult; it hardly forms at birth more than the middle third of the bone, the superior and inferior thirds being formed at first by the cartilaginous portion of each extremity, then a texture of cells intermediate between this portion and the canal; so that as we advance in age, its length becomes in proportion greater.

II. Of the Flat Bones.

The flat bones have in general, but little relation to locomotion, which they only assist by the insertion of the muscles that go to the long bones. Nature designs them especially to form the cavities, such as those of the cranium and the pelvis. Their conformation renders them very proper for this use. Their number varies according to the cavities with which they are connected; many always unite to form one, and it is this circumstance that contributes in part to their solidity. In fact, external blows losing their force at the place of their junction, fracture them with less ease. If the cranium was only one single piece, its solutions of continuity would be much more frequent than they now are. So that as the sutures ossify in old age, they become more brittle. In children, in whom the ossification is not complete, and the number of whose separate, osseous pieces is consequently more considerable in the head, the pelvis, &c. the difficulty of fractures is very great, because the soft bands which unite the solid parts yield to external bodies, without breaking.

The flat bones are almost all curved, concave and convex on the opposite sides; this arises from their destination in the formation of cavities. Their curve varies according to the place in the cavity they occupy; this curve is the cause of a very powerful resistance, when that mentioned above does not exist. Thus in the first age, the cranium resists by yielding; but as the sutures become more closed, and only one osseous piece is formed, it is by the mechanism of the arch that the brain is protected.

All the flat bones have two surfaces and a circumference. According as the first serve for muscular insertions, or are only covered by aponeuroses, membranes, &c. they are rough or smooth. Towards the middle the bone is thinner; it has more thickness at the circumference, which is either for articulation or insertion. In the first case, this excess of thickness gives more solidity to the joints, which are then made with larger surfaces, as we see in the cranium; in the second, it presents to the fibres more points of origin, as we see on the crista of the ilium and the greater part of its circumference.

The internal forms of the flat bones have but few peculiarities; their two external layers leave between them a space which is filled by the texture of the cells.

III. Of the Short Bones.

The short bones are placed in general in parts where are found united mobility and solidity, as in the vertebral column, the tarsus, and the metatarsus. Always small, they are in great number in the regions which they occupy; their number compensates for their size in the formation of the parts of the skeleton to which they contribute. It is this number also, that gives to these parts the union of the two almost opposite attributes of which we have spoken, viz. solidity, because the external efforts are lost in the numerous bands which unite them, and mobility, because the whole of their individual motions gives a considerable general motion.

There is nothing constant or uniform in the external conformation of these bones; it is modified according to the general plan of the whole, of which they are the parts; thus the different uses of the carpus, metacarpus and vertebral column determine the different forms of their respective bones. These bones have always many cavities and eminences upon their external surfaces, necessary for their numerous articulations, for the insertion of the many ligamentary cords that unite them, and the muscles that move them.