1st. The order of immoveable articulations with surfaces in juxta-position, is met with where the mechanism of the part alone is almost sufficient to insure the solidity of the bones which are found only placed at the side of each other, without holding by any insertion, and having only between them a slight cartilaginous layer; the superior maxillary bones, wedged in between the malar bones, the ossa ungues, the ethmoid, the ossa palati, the vomer, and the frontal bone, are supported more by the general mechanism of the face, than by any articular attachments that unite them to each other; thus the squamous portion of the temporal bone supports the parietal, more by the abutting arches, than by the manner of the union of their respective surfaces. Remove this general mechanism of the part, you will soon see all the articulations separate. 2d. The order of immoveable articulations with inserted surfaces, owes also in some measure its solidity to the general mechanism of the part; but this mechanism would be insufficient to insure this solidity; thus the bones, instead of having almost plain surfaces, exhibit very evident prominences and depressions which are inserted into each other, as we see in the articulations of the parietal bones with each other, with the sphenoid, the occipital, the frontal, &c.; these are called sutures. This order sometimes approximates the preceding, as in the union of the parietal and frontal bones, which, reciprocally aiding each other, are supported by this mechanism, more than by their insertions; sometimes it resembles the following order, as in the articulation of the occipital and parietal bones, in which the very deep insertions almost alone insure the solidity of the union. This order is never seen except upon the edges of the flat bones; the insertion of these edges compensates for their want of size, by multiplying the points of contact. The eminences and depressions forming the insertion are always of an irregular form and size. They are exactly fitted to each other, they are not alike in two bones of the same species, taken from two different subjects; so that we cannot unite to a detached left parietal bone, the right parietal bone of another individual. There has been much dispute upon the formation of the sutures; they are an effect of the laws of ossification, an effect which we can account for no more than we can for all the others, and all the general phenomena of growth; we shall see the progress they follow in this formation. This articular order is gradually effaced with age, and the bones unite together by the ossification of the thin intermediate cartilage. It is more rare that the preceding order disappears. I have seen, however, in extreme old age, different articulations of this order cease to be evident, those of the maxillary bones between themselves especially.