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The cellular lining of the ventricular system and central canal of the spinal cord.

A choroid plexus is a vascular and epithelial structure projecting into a brain ventricle and contributing to cerebrospinal fluid production. Its epithelial barrier separates blood-associated and ventricular compartments. Secretion is a regulated transport process rather than simple unfiltered leakage from vessels.

The cerebral aqueduct is the narrow midbrain passage connecting the third and fourth ventricles. Its small caliber makes the relation between geometry and fluid passage important. It is a conduit rather than the principal site of cerebrospinal-fluid production.

The central canal of the spinal cord is the narrow cavity along the central spinal region, developmentally continuous with the ventricular system. In adults its patency varies. Anatomical continuity does not imply that it carries most cerebrospinal-fluid outflow from the fourth ventricle.

Venous thrombosis involving deep veins, commonly in a leg. Local obstruction and the possibility of embolization are distinct consequences of the same clot.

A disease category encompassing deep vein thrombosis and pulmonary embolism. It connects clot formation in veins with the possible transport of clot material into pulmonary arteries.

Chronic symptoms and tissue changes after venous thrombosis, associated with persistent obstruction or damaged venous valves. The later syndrome can remain after the original acute clotting event has ended.

Adding a spatially constant function of time to a velocity potential leaves its velocity $\mathbf{u}=\nabla \varphi$ unchanged. When integrating the Euler equations in space, this freedom can absorb the time-only integration term in the potential form of momentum balance. A spatially constant zero-frequency potential represents no acoustic velocity and must not be counted as a propagating normal mode merely because a frequency determinant vanishes at zero. Pressure conventions and time dependence must still be fixed consistently when the potential is used to recover physical fields.

A structure within a vein that helps oppose backward movement of blood. William Harvey used observations of these structures and experiments with ligatures in his argument for blood circulation. ^[ref]

The superior ophthalmic vein is a major venous drainage route from the orbit toward the cavernous-sinus region. Its connections with anterior orbital and facial veins provide multiple communicating routes. Venous drainage should be distinguished from the neighboring ophthalmic arterial supply.

The inferior ophthalmic vein drains portions of the orbit through connections that vary among individuals. It communicates with deep facial or cavernous-sinus routes. Its name identifies a drainage pathway rather than a vessel supplying oxygen to the retina.

The cavernous sinus is a paired cranial venous space beside the pituitary region. It has close relations to the internal carotid artery and several ocular motor nerves. Those relations explain why one localized process can produce combined vascular and neurological effects.

The pterygoid plexus is a venous network in the deep facial region around the pterygoid musculature. Connections link it with facial and intracranial drainage routes. Network connections help explain how neighboring spaces can communicate despite different superficial locations.

Phlebitis is inflammation of a vein. It names an inflammatory process rather than proving thrombosis in every case. A clot-associated process and isolated wall inflammation are related but distinct pathological descriptions.

The azygos vein is a major posterior thoracic venous channel, usually ascending on the right and joining the superior vena cava. It links segmental drainage and collateral routes. The pathway therefore matters beyond merely emptying one local tissue region.

The subclavian vein continues upper-limb venous return across the thoracic inlet and joins the internal jugular vein to form a brachiocephalic vein. Its relation to the clavicle and nearby vessels is important for regional anatomy.

The deep dorsal vein of the penis drains erectile and adjacent structures along a deep dorsal route. Its connections include pelvic venous pathways. It is distinct from superficial veins draining skin and superficial tissues.

A dural venous sinus is a venous channel associated with layers of cranial dura. It receives intracranial drainage and connects toward the internal jugular route. Its wall and location differ from an ordinary freely mobile vein in soft tissue.

A vena cava is a major systemic venous channel returning blood to the right atrium. The superior and inferior venae cavae drain different body regions. Their return reflects upstream volume and pressure relationships rather than active pumping by the vein itself.

A lumped venous reservoir stores blood at a pressure determined by its volume and vascular compliance. If its inflow and outflow are $Q_{\mathrm{in}}$, $Q_{\mathrm{out}}$, the constant-compliance approximation gives $C_v{\dot{p}}_v=Q_{\mathrm{in}}-Q_{\mathrm{out}}$. It forms the filling side of a three-compartment ventricular circulation model.