It appears then that it is the membrane itself, and not a fluid that escapes from it, which serves to protect the artery; it can, in this point of view, be considered in relation to the blood, as a kind of epidermis. It is this, which by its folds contributes especially to form the aortic and mitral valves, and the different eminences at the origin of the branches, smaller branches, &c. The external surface, feebly united to the other membrane as we have seen, has not an intermediate cellular one. Notwithstanding this slight adhesion, no means, boiling water, maceration, putrefaction, &c. can detach one of these membranes from the other, as takes place in the periosteum from the bone, which, are naturally much stronger united; it requires always the aid of dissection.
What is the nature of this common membrane? I am entirely ignorant; though with a different appearance it has the greatest analogy with the preceding coat, in its properties. We cannot class them in any system. They form a separate texture in the economy, a texture that has properties entirely distinct.
When we dry the common membrane of the arteries by itself, it is infinitely more pliable than the other. It remains transparent, the other does not. As to the phenomena of the other re-agents, except the horny hardening, they are nearly the same.
This membrane is remarkable, in all the organic systems, for the singular tendency it has of being ossified in old age. I have been able to satisfy myself, that out of ten subjects, there are at least seven that have incrustations after the sixtieth year. These incrustations, having no connexion with the peculiar fibrous membrane, begin uniformly on the external surface of this, the most external part of which they attack; for there always remains over the incrustation a kind of pellicle that separates it from the blood, and which belongs to the membrane; the earthy substance is never immediately in contact with this fluid.
These incrustations do not follow any of the laws of ordinary ossification. The cartilaginous state rarely precedes them. The saline substance is deposited immediately upon the exterior of the common membrane by the exhalants. It is always in separate plates, more or less broad, that this exhalation is made; rarely the whole of the artery forms a solid continuous tube; so that the membranous portions remaining between the plates can be considered as serving for articular connexions, and that the arteries, thus ossified, are composed of many pieces moveable upon each other, and being able to a certain extent to adapt themselves to the circulating motion.
As long as these plates remain thin, the interior of the artery is as usual smooth and polished. But if many saline substances are deposited there, they then have a greater thickness and make a projection within. The fine pellicle that covers them and which is continued upon the artery, is broken; then they adhere only by their external surface to the peculiar membrane. Their circumference is unequal and rough. If there is a great number in the artery, the whole internal surface presents numerous asperities, produced by the rupture of this extremely fine layer of the common membrane that covers the osseous plates. This arrangement is particularly remarkable at the origin and even in the course of the aorta. I have noticed it frequently in the dissecting rooms. Since I have practised medicine in the hospitals, I have already opened three or four subjects that have exhibited this arrangement, in which the heart was perfectly untouched, but who died however with most of the symptoms that accompany diseases of that organ. The rupture of the fine pellicle when the osseous plates become large, arises from the remarkable brittleness that we have observed in the common membrane, of which it is an appendage. I have never seen these osseous plates entirely detached, and become loose in the artery.
All the parts of the arterial system are subject to ossification. It appears as frequent in the branches as in the trunks. We know how common it is to find the radial ossified, in feeling the pulse of an old person. The ramifications appear to be less frequently the seat of these incrustations, which never take place in the capillary system; a circumstance that would induce me to believe that the common membrane of the arteries does not extend to this system, but that it changes gradually into a different texture.
It is not only in the arteries that the common membrane of the system with red blood is penetrated with saline substance; this often happens to it in the heart, especially in the aortic and mitral valves. It is more rare upon the internal surface of the left ventricle and auricle and the pulmonary veins. I have had however examples of these last. This general disposition to ossification in its whole course, proves that its nature is every where the same, and that notwithstanding the differences pointed out, I have had reason to consider it in an uniform manner, from the pulmonary capillary system to the general; for as I have already observed, an identity of diseases supposes an identity of nature. It is the frequency of ossifications of this membrane in the heart of old people, which renders extremely frequent the intermission of the pulse at that age. The ossification of the origin of the aorta has an influence also upon the circulation, as I have ascertained; but that of the trunks, branches, &c. does not produce the least derangement.
The ossification of the common membrane of the system with red blood differs essentially from those that happen in other parts, in this, that it is, if we may so say, a natural phenomenon, whereas the others are accidental and often preceded by inflammation and engorgement. Thus these ossifications do not follow the progress of age; they happen in young people and in adults, as often as in old ones. Before old age, the ossifications of this membrane are observed also, but infinitely more rarely than at this age. The diseases of the heart which the ossification of the mitral valves accompanies and often alone constitutes, are a remarkable proof of this. A phenomenon has struck me many times upon this subject; such an ossification as an old man can live with very well, and which only makes his pulse intermittent, produces in the adult the most serious consequences. I have already opened many subjects, who had been affected with difficulty of breathing, frequent suffocation, cough, irregularity of the pulse, necessity of an erect position of the trunk, and in the later periods, infiltration, serous effusion in the thorax, spitting of blood, &c. and in whom I have found only ossification of the mitral valves, less than we see every day in the bodies of old people in our dissecting rooms. I confess that even this natural disposition to ossification in the common membrane of the system with red blood, had made me think that they had exaggerated a little the cases in which this ossification becomes, in the adult and even in the old man, when it is very strongly marked, the cause of that series of phenomena, whose assemblage forms the asthma of most physicians. But the practice of the Hôtel Dieu shows me every day, that these cases of ossifications, those of aneurisms and those of other organic affections of which the heart is the seat, form a class of chronic diseases almost as numerous as that of the chronic diseases of the lungs, to which generally were referred all the diseases of the chest, before the time of Corvisart.