Plexus

PLEXUS, plexus, an anatomical concept referring to the peripheral sections of the nervous, vascular, and lymphatic systems in cases where a loop-like system of connecting branches forms between the trunks of several corresponding nerves or vessels in specific, more or less limited areas. In essence, in the vascular and lymphatic systems, a plexus represents a system of anastomoses, but differs from ordinary anastomoses by the abundance and diverse character of the connections concentrated in more or less limited areas. A distinguishing property of the vascular plexus is also the circumstance that the trunks connected by means of the plexus lie at different depths, by virtue of which the entire plexus is not located in a single plane. In this, a vascular plexus differs from the so-called vascular network (rete), in which the system of connecting branches is located more or less in a single plane. The vascular plexus also differs from a network in that within a plexus the connecting vessels have a more or less uniform caliber, whereas in a network the vascular branches coming from various trunks have a gradually narrowing caliber and possess the function of collaterals. In general, however, the distinction between a plexus and a network is only of a conventional and relative character. Venous plexuses are especially developed in the pelvic region, along the vertebral canal, and at the base of the skull (plexus pterygoideus sup. et inf., plexus vertebralis, plexus foraminis ovalis, etc.). The significance of the vascular plexus lies in the uniform distribution of blood in a given area. Of special importance is the choroid plexus of the brain ventricles (plexus chorioideus), formed by vessels of the pia mater growing into the cavity of the ventricles. This plexus is covered with a cubic glandular epithelium participating in the secretion of cerebrospinal fluid. In relation to nerve plexuses, the notion of anastomoses is incorrect (Schaffer), because nerve fibers have no lumens and in nerve plexuses there is usually no fusion of nerve fibrils originating from various nerve trunks participating in the formation of the plexus, but merely the attachment of individual bundles of nerve fibers from one trunk to another or the mutual exchange of individual fibers between trunks (see Anastomoses); at the same time, some fibers may acquire a direction opposite to the course of the entire trunk (different direction of excitation conduction). Thanks to the formation of plexuses, the path of the nerve fiber is significantly lengthened, and the fiber reaches the organ not by the shortest path, but by a roundabout way through a multitude of bundles. Nerve plexuses are widely distributed in all parts of the nervous system (about 100 plexuses). Thanks to nerve plexuses, a more complete innervation of organs and tissues occurs.

D. Vyropaev.