Sinus - a curved out hollow space in side the skull which is lined by respiratory epithelium and drains into the back of the throat. For example, maxillarys., ethmoid, s., sphenoids.

Solubility-product - a value found by multiplying the concentration of positive IONS by the concentration of negative ions in a solution of a salt, hence [Ca]+ x [P04]- = Ksp(solubility product). The value for Ksp is constant when the solution of ions is saturated and in balance with its solid crystalline form. Acid helps increase the solubility of a weakly soluble salt.

Sphenomandibular ligament - a ligament which joins the lingula of the mandible to the spine of the sphenoid bone.

Spongy-bone - the bone beneath the cortical bone which has been thinned out by bone remodelling to form a spongy inner core. Also called cancellous (lace-like) bone.

Stem cells - cells from which a number of more Stimulated saliva - saliva which has been stimulated by chewing.

Stippled - a pattern which is made of small dots. Gingiva has a stippled appearance due to small depressions caused by the attachment of clumps of fibres in the lamina propria to the basement membrane6 of the epithelium..

Striated7 ducts - ducts which carry saliva from the intercalated ducts to a series of main collecting ducts. Striated duct cells are actively involved in secretion and absorption. Their striated appearance is due to the many long folds of the cell membrane.

Submucosa - a layer beneath the lamina propria of mucosa, which is loose and elastic. It may contain large blood vessels, nerves, glands and lymphatic tissue.

Substance P - a neuropeptide with a particularly powerful ability to excite a post synaptic cell. When substance P is released into a synapse of a sensory neurone it causes severe pain.

Sulcus - see Gingival sulcus.

Super-saturated solutions - When a solution is saturated and still more ions are added, they cannot be held in solution but precipitate as a solid deposit. The proline-rich proteins of saliva are capable of binding calcium. They hold a store of calcium ions which allows saliva to carry more ions in solution than is theoretically possible. Saliva is thus a super-saturated solution of calcium phosphate.