It is to this softness, this want of resistance of the fibrous system in the first years, that the following phenomena must be attributed. 1st. The articulations yield at this age to motions which the stiffness of the ligaments afterwards renders impossible; all extensions can then be carried beyond their natural degree. We know that it is at this period that tumblers begin to practice; they would never be able to execute those extraordinary motions, which astonish us, if habit did not preserve in them from infancy the power of these motions. 2d. Luxations are in general rare in the first age, because the fibrous capsules yield and do not break. 3d. Sprains have then less serious consequences. 4th. The inflammatory swellings under the aponeuroses are rarely susceptible of those strangulations oftentimes so severe at the adult age. 5th. This softness of the fibrous system accommodates itself also in the tendons, the ligaments, the aponeuroses, &c. on the one hand to the multiplicity and frequency, and on the other to the want of power of the motions of the infant.
I would remark, that although the fibrous system has in the first age a softness of texture nearly uniform in all the parts that belong to the same order, yet it is more or less developed according to the regions in which it is found. In general, when it belongs to the organs that are early developed, as to the brain by the dura-mater, to the eyes by the sclerotica, &c. it has in proportion more size and thickness; but it is only in its dimensions, and not in its intimate organization, that these differences then exist.
It is probable that this mode of organization of the fibrous system has an influence, at the period of which we are treating, upon its degree of vitality and consequently upon its diseases. We know that rheumatism, which appears very probably to affect this system, is rarely the attendant of children of the first age; that in a hundred patients affected with it, there are ninety at least above the age of fifteen or sixteen years.
Subjected to ebullition, the fibrous system of the fœtus and the infant easily melts, but does not take that yellow colour, which it constantly has, when boiled in the adult age; we know that the jellies made from young animals are much whiter than those from older ones.
II. State of the Fibrous System in the after ages.
As we advance in age the fibrous system becomes stronger and more compact; it remains stationary in the adult age, though the alternate absorption and exhalation of nutritive substances constantly continue. These two functions can scarcely be seen in the ordinary state; but the first is very apparent when from a contusion or any internal cause, the periosteum, the fibrous capsules, the ligaments, &c. swell. The second in its turn predominates, when the swelling subsides and resolution takes place.
In old persons, the fibrous system becomes more and more compact and contracted; it yields more slowly to maceration and putrefaction. The teeth of animals that feed upon it, tear it with more difficulty; the gastric juices act upon it less easily. Spallanzani has observed, that the tendons and aponeuroses of old animals were much more indigestible than those of young ones. With age, the force of the fibrous texture increases; but its softness diminishes; hence the difficulty of the motions, from its stiffness. The ligaments and the fibrous capsules do not allow the articular surfaces to separate easily from each other; the tendons bend with difficulty; when we pass externally on places where they are directly under the integuments, we perceive that they are hard, not supple, &c. It requires a long time to soften them by ebullition. The whole fibrous system becomes yellow. We should say that it approached then that state in which it is compact, semi-transparent and has the horny hardening to which desiccation reduces it; so that if we could suppose this system going through quicker than the others the different periods of its decrease, all the motions would cease from the rigidity of the ligaments, tendons and aponeuroses, though the energy of contraction might still subsist in the muscles.
III. Preternatural Development of the Fibrous System.
We have seen that different productions belonging, by their nature, to the osseous or cartilaginous systems, are sometimes preternaturally developed in certain parts. Morbid anatomy also shows us productions, in which the fibrous appearance is very evident. I have many times made this observation in tumours of the womb, the fallopian tubes, &c. Instead of the lardy matter which is so common in these organic affections, we see one or several masses of fibres, very distinct, yellow, &c. I cannot however say that these excrescences belong essentially, by the substances that compose them, to the fibrous system, not having made upon them, experiments similar to those which I have made upon the organs of this system.