Canal of Schlemm
Sinus venosus sclerae
Also known as: Schlemm canal, scleral venous sinus, canal of Schlemm
A circular, endothelium-lined channel encircling the limbus that collects aqueous humour from the trabecular meshwork and passes it to the episcleral veins.
Overview
The canal of Schlemm runs circumferentially in the scleral sulcus at the corneoscleral junction. It is a modified, lymphatic-like vessel: its inner wall endothelium faces the juxtacanalicular meshwork and transports aqueous by forming giant vacuoles and transcellular pores in response to the pressure gradient; its outer wall drains via ≈25–35 collector channels to the deep and intrascleral venous plexuses and aqueous veins, which join the episcleral veins.
Because the canal normally contains aqueous rather than blood, the gradient from intraocular pressure (≈15 mmHg) to episcleral venous pressure (≈8–10 mmHg) keeps blood out; when episcleral venous pressure rises (e.g. carotid–cavernous fistula, Sturge–Weber syndrome) blood may reflux into it and IOP rises.
The canal is the final common target of several glaucoma operations (canaloplasty, micro-stents/MIGS) that aim to bypass the trabecular meshwork.
- Location
In the scleral sulcus at the limbus, external (outer) to the trabecular meshwork and anterior to the scleral spur.
Anatomy
- Histology
Single layer of endothelium (inner wall with giant vacuoles, discontinuous basement membrane) expressing lymphatic markers such as PROX1; thin connective-tissue wall.
- Development
Forms late in gestation from scleral venous plexus by a process resembling lymphangiogenesis; maldevelopment of the angle causes primary congenital glaucoma (buphthalmos — enlarged "ox eye", cloudy cornea, photophobia, epiphora).
Function
- Collect aqueous humour from the trabecular meshwork
- Return it to the venous circulation via collector channels and episcleral veins
Clinical significance
Congenital glaucoma
Angle dysgenesis prevents normal outflow; because the infant sclera is elastic, raised IOP enlarges the globe (buphthalmos) and stretches the cornea (Haab striae). Treated surgically (goniotomy, trabeculotomy).
Test yourself
Through which route does most aqueous humour leave the eye?
- Uveoscleral outflow through the ciliary muscle
- Posterior diffusion through the vitreous to the retinal veins
- Across the corneal endothelium into the tear film
- Trabecular meshwork → canal of Schlemm → episcleral veins
Answer: Trabecular meshwork → canal of Schlemm → episcleral veins
About 80–90% of aqueous drains by the conventional (trabecular) pathway into Schlemm canal and then to aqueous and episcleral veins; ≈10–20% leaves by the uveoscleral route, which prostaglandin analogues enhance.
Compiled from standard anatomy and physiology references — see the Special Senses sources. For education only; not medical advice.