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    Saturated solutions - Salts such as the apatites do not readily become ionized and dissolve in water. When no more ions can dissolve the solution is said to be saturated. The concentration of ions in a saturated solution, its solubility product, is constant for each salt, at a neutral pH. If the solution becomes more acidic, more ions can dissolve from the solid. Saliva is a super saturated solution of calcium phosphate.

    Schwann cells - members of a family of nerve-supporting (neuroglial) cells. The Schwann cell has an extensive cy lasm which allows it to wrap a myelin sheath around nerve axons.

    Sclerotic - hardened, as in sclerotic dentine, which is hardened by intratubular dentine in response to tooth wear, ageing and arrested caries.

    Secondary caries - caries which has occurred after a primary lesion has been restored. It is most commonly due to failure of the restoration at its margins, which have broken down and allowed a leak to develop between the restoration and the wall of the cavity. Caries bacteria which have been left behind during cavity preparation are unable to produce secondary caries if the margins of the restoration have achieved a proper seal against the tooth.

    Septa - the thin plate of bone between the roots of teeth ( Latin septum = a wall). Also the fibrous walls which separate sections of a gland. Septa separate sections of an orange or grapefruit.

    Serotonin - ( 5 Hydroxytryptamine) is present throughout the body, especially in blood platelets and in the intestines. Its release from blood platelets contributes to the pain, vasoconstriction and inflammation after injury. In nervous tissue it functions as a neurotransmitter, mainly in the midbrain in clusters of cells called the raphe, and in the medulla. The fibres of these cells connect with the forebrain, cerebellum and spinal cord. It therefore exerts a strong influence over arousal, sensory perception, emotion and thought. Drugs which slow down the removal of serotonin can reduce depression and pain.

    Serous - a watery secretion which resembles serum.

    Serum - the fluid component of blood from which the clotting protein fibrinogen has been removed.

    Sesamoid bone - a small bone which appears at the age of thirteen, adjacent to the carpo-metacarpal joint of the thumb and is of use in determining the skeletal age of a child.

    Sharpey's fibres. - are collagen fibres which have been trapped in bone or cementum in order to anchor them. see also extrinsic fibres.

    Sialoproteins - a family of adhesion molecules which include osteopontin. Bone sialoprotein is formed by cells lining the root surface and influences cementoblast differentiation which encourages mineralisation. Dentine sialoprotein appears to inhibit mineralisation. Osteoclasts adhere both to bone sialoprotein and osteopontin.

    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, maxillary s., 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 membrane of the epithelium..

    Striated 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.

    Symbiosis - a mutually beneficial inter-relationship between two organisms, for example between bees and flowers (pollen carrying in return for nectar).

    Sympathetic nerves - the sympathetic nervous system is one of he two main divisions of the autonomic nervous system. Sympathetic nerves make synapses at ganglia close to the spinal cord and have long post -ganglionic axons which, in general place the body on a state of alert. Always acts as a balance to activity of the parasympathetic nervous system.

    Synapse - the point at which one nerve cell connects with another. The nerve impulse is transmitted by the release of chemical neurotransmitter substances from the pre-synaptic cell membrane. The neurotransmitter substance diffuses across the synaptic cleft to the post-synaptic membrane which it depolarises. When the post-synaptic cell is sufficiently excited by a number of incoming impulses and enough neurotransmitter substance, it discharges an electrical impulse along its axon membrane to the next neurone.

    Synovial fluid - the lubricating fluid containing glycosaminoglycans which is held in the capsule of a synovial joint.

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