The plexuses of veins are merely a high degree of anasto mosis : they are not subservient to any laws in their formation and arrangement, or susceptible of any systematic division ; and will probably be best under stood by describing and figur ing a few of the most charac teristic.
The simplest and most primi tive form of plexus is that which is produced by the inosculations and joinings of two vence coniites around and across an artery. Such is exhibited in the accom panying figure : the anastomoses are so numerous, that it is diffi cult to tell in parts whether they are to be considered as two vessels, or as one, with fre quent trivial interruptions of cavity : at some points they run a considerable distance without conjunction, leaving an elongated elliptical interval ; and at other points 'their confluence is so great that the intervals are re duced to small circular apertures on the face of one broad vein.
A coarser and more compli cated form of plexus is exhibited in the subcutaneous system of veins, in the production of which several venous trunks combine, and by their lateral branchings and confluxes produce diamond shaped, rhomboidal, and triangular inter spaces : — these anastomoses being all on the same plane, excepting at a few points here and there where a small channel dips down to the deep veins. Such plexuses may be seen on the dorsum of the hand and foot.
But the most elaborate and complex of all the plexuses in the human* subject are those formed around, about, and within, the spinal canal: they are composed of nu merous trunks, which unite, divide, and re unite, at every possible point, and in all con ceivable modes, by branches of all sizes, lengths, and shapes, and leave intervals pre senting forms of endless variety. A portion
of these plexuses, seen in the accompanying figure, from Breschet's work on the veins, conveys a better idea of them than any lengthened description.
The diploic plexuses are the net-works of veins which exist in and among the cancel lated tissue of the bones. In the flat bones of the cranium, at the period of adult life, they form large irregular meshes, by the mean derings of large, irregular, ampullated, veins. These vessels are very unequal in size, are subject to dilatations, and frequently end in culs-de-sac. They are well represented in figs. 187. and 188. Vol. I. But the most re markable peculiarity in these plexuses, is the change they undergo during osseous de velopment. In early fcetal life, when ossifi cation commences the cranial bones consist of stellm of numerous ossific rays, the inter. spaces between them being occupied by host: of small, almost straight, radiating, veins: these veins are not covered in by osseoue structure, but are exposed on both surface: of the bone (fig. 872.); they then gradually become tortuous, and fuse into one another so as to diminish in number, and lose much of their radiating character ; and after a time they become covered by a thin plate of bone on either surface. The process of fusion ant dilatation of the veins still goes on Burin; r life, and ultmately leaves the diploic plexus consisting of a few large vessels. Diploie vessels exist in all cancellated bone in various plexiform combinations. In the loose texture of the bodies of the vertebrae remarkable plexuses exist. (Fig. 87 1.)