Besides, that which shows the vital energy in an organ, is the rapidity with which inflammation goes through its periods in it, and the frequency of this affection, &c. Now in the bones inflammation is so much the more rapid, in proportion to the greater quantity of cartilaginous texture they contain; observe the periods of the formation of the callus in the different ages, periods which are determined by the duration of the inflammation necessary for its formation, you will see that in infancy they are short, that they are much longer in old age, and that often even consolidation does not take place, whilst it is effected with facility in all the soft parts. The general weakness no doubt of all the vital forces which takes place from the effect of age is one cause of this slowness and this rapidity in the formation of the callus at the two extreme periods of life; but the different proportions of gelatinous and calcareous substances contribute much to it also; for when we compare other cicatrices with this, the cutaneous, for example, we see that age establishes in them an infinitely less sensible difference as it respects the slowness and rapidity of this reunion, than in the osseous system. The bones have not sufficient life to inflame and unite, like the skin, the muscles also exhibit this phenomenon in a very evident manner. I have seen an old man, the neck of whose fractured thigh-bone remained a long time without reunion, and in whom a wound of the face was healed very speedily by the first intention.
Finally, there is a simple experiment that I have often made, and which proves as well as the preceding facts, that the cartilage is truly the animal part of the bone. We know that one of the great attributes of animal substances, is to contract and exhibit the horny hardening when burnt; now when the bone is penetrated with its earthy salt, it has not this kind of combustion; deprived of this salt by an acid, the cartilaginous parenchyma which remains burns in this manner. The flat bones in infancy in which this parenchyma predominates, exhibits also this phenomenon in burning; it forces the calcareous portion, which is in small quantity to obey the impulse that it gives it, and turns it in various directions; but in the adult in whom the calcareous portion is the largest, the bone remains unmoved while the fire penetrates it, and its whole cartilage is taken away, without its fibres being able to obey their tendency to the horny hardening which combustion imprints upon them.
ARTICLE FOURTH.
OF THE ARTICULATIONS OF THE OSSEOUS SYSTEM.
All the bones are united together, and thus form the skeleton. The manner of their union varies, but whatever it may be, it is known under the general name of articulation.
I. Division of the Articulations.
All the articulations can be referred to two general classes. Mobility is the character of the first, immobility that of the second.
One belongs to all the bones which serve for locomotion, to some of those destined to internal functions, as the ribs, the lower jaw, &c. The other is especially met with in the bones, the union of which forms the cavities designed to defend the organs; this we see in the head, the pelvis, &c.
Moveable Articulations. Observations upon their Motions.
I divide moveable articulations into four kinds, the characters of which are borrowed from the different motions they execute. To understand this division, it is necessary previously to know the motions of the articulations in general. These motions can be included under four species, which are, 1st, opposition; 2d, circumduction; 3d, rotation; 4th, sliding.