GENERAL OBSERVATIONS.
The former part of this work has been devoted to researches upon the systems common to the structure of all the apparatus, upon the primitive systems, which form if we may so say the nutritive parenchyma, the basis of all the organs, since there is hardly any one of these organs in which the arteries, the veins, the exhalants, the absorbents, the nerves and the cellular texture do not enter as a more or less essential part. Each is at first a texture of these common parts, then of other peculiar parts which particularly characterize them.
The systems that will now be examined are not so generally extensive in the animal economy. They belong only to some particular apparatus; thus the osseous, the animal muscular, cartilaginous, and fibrous systems are especially destined to the apparatus of locomotion; thus the serous, mucous, and organic muscular systems enter especially into the digestive, respiratory and circulatory apparatus; thus the glandular system forms the apparatus of secretions, thus the cutaneous system enters principally into the external, sensitive apparatus, &c.
All the systems that remain to be examined are then much more insulated, perform a much less extensive part than those of which we have been treating. Confined to certain apparatus, they are unknown to the others, and have an independent life of their own, whereas the primitive systems everywhere mingle their vitality with that of the other organs, into whose composition they enter; most of them have a kind of existence and external forms which distinguish them from these last. The different parts which compose each, are almost always insulated, not connected with each other; the bones, the muscles of animal and organic life, the cartilages, the fibro-cartilages, the medullary organs, the glands, the serous membranes, the hair, &c. exhibit this insulation in a remarkable manner. Each portion belonging to these different systems, has always between it and the other portions of the same system many intermediate organs, which are of a very different nature, and which consequently belong to other systems. There are hardly any except the cutaneous, fibrous and mucous systems, which are everywhere continuous in their different parts; yet this last has no communication between that portion of it which is spread upon the digestive and respiratory apparatus, and that which belongs to the urinary and genital organs.
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.