sacro-iliac joint - synovial joint formed between the medial surface of the ilium and the lateral aspect of the upper sacral vertebrae; a fibrous joint, comprising and surrounded by very strong ligamentous structures; major function is to transmit body weight, but also has a

saline solution - a solution containing sodium chloride.

saliva - mixture of water, protein, and salts that makes food easy to swallow; begins digestion.

salpingectomy - surgical removal of one or both fallopian tubes.

scapula - commonly known as the shoulder blade.

sciatica - traditionally defined as pain in the sciatic nerve, or its branches, that is caused by direct pressure or irritation.

sclerotherapy - method of stopping upper gastrointestinal bleeding. A needle is inserted through an endoscope to bring hardening agents to the place that is bleeding.

scotoma - an area of partial or complete loss of vision surrounded by an area of normal vision.

sebum - oily substance produced by sebaceous glands in the skin.  

secretin - hormone made in the duodenum; causes the stomach to make pepsin, the liver to make bile, and the pancreas to make digestive juices.

segmental mastectomy - surgery to remove a portion of the breast.

sella turcica - bony structure that houses the pituitary gland.

sensorineural hearing loss - hearing loss caused by damage to the sensory cells and/or nerve fibers of the inner ear.  

septal defect - a hole in the wall of the heart.

septum - the muscle wall that divides the heart chambers.

serum - a clear fluid that separates when blood clots.

sesamoiditis - inflammatory condition affecting the two sesamoid bones in the flexor tendon of the great (big) toe.

sexually transmitted disease (STD) - infection spread through sexual intercourse and other intimate sexual contact.  

shigellosis - infection with the bacterium Shigella; usually causes a high fever, acute diarrhea, and dehydration. (See also gastroenteritis.)

shunt - a connector that allows blood flow between two locations.

Shwachman's syndrome - digestive and respiratory disorder of children in which certain digestive enzymes are missing and white blood cells are few.

sialorrhea - drooling.

sigmoid colon - lower part of the colon that empties into the rectum.

silent ischemia - ischemia not accompanied by chest pain.

simple mastectomy - surgical removal of the breast and possibly a few of the axillary lymph nodes close to the breast.

sinus (Also called paranasal sinuses.) - air cavities within the facial bones, lined by mucous membranes similar to those in other parts of the airways.

sinus node - the cells that produce the electrical impulses that cause the heart to contract.

slump test - a test for assessing the mobility of the pain-sensitive structures in the vertebral canal. 

small intestine - organ where most digestion occurs; measures about 20 feet and includes the duodenum, jejunum, and ileum.

smell - to perceive odor or scent through stimuli affecting the olfactory nerves.

smooth muscle - muscle that performs automatic tasks, such as constricting blood vessels.

soft-tissue - generally, the ligaments, tendons, and muscles in the musculoskeletal system.

somatosensory - refers to sensory signals from all tissues of the body including skin, viscera, muscles, and joints.

somatostatin - a hormone in the pancreas that helps tell the body when to make the hormones insulin, glucagon, gastrin, secretin, and renin.

spasm - a condition in which a muscle or group of muscles involuntarily contract.

spasmodic dysphonia - momentary disruption of the voice caused by involuntary movements of one or more muscles of the larynx (or voice box).

spastic colon - see irritable bowel syndrome (IBS).

speech - making definite vocal sounds that form words to express thoughts and ideas.

speech-language pathologist - health professional trained to evaluate and treat people who have voice, speech, language, or swallowing disorders, including hearing impairments, that affect their ability to communicate.

sphincter - ring-like band of muscle that opens and closes an opening in the body.

sphygmomanometer - the instrument used to measure blood pressure.

spinal anesthetic - an anesthetic which is injected into the spinal canal fluid for surgery in the lower abdomen, pelvis, rectum, or other lower extremities.

spinal instability - increased motion between vertebra, usually resulting from an injury; pain typically feels like tingling in the neck or arms.   

spine - a column in the body consisting of 33 vertebrae.

spirogram - record of the amounts of air being moved in and out of the lungs.

spleen - organ lying between the stomach and diaphragm that stores red blood cells and filters blood.

splenectomy - surgical removal of the spleen.

splenic flexure syndrome - condition that occurs when air or gas collects in the upper parts of the colon.

spondylosis - a condition of the spine involving a degenerative process.

sputum (Also called phlegm.) - mucous from the lungs.  

steatorrhea - condition in which the body cannot absorb fat.

stenosis - the narrowing or constriction of a blood vessel or valve in the heart.

stereopsis - ability to perceive three-dimensional depth.

sternum - the breastbone.

stethoscope - the instrument used to listen to the heart and other sounds in the body.

stirrups - technique of ankle strapping using rigid tape placed on the ankle; medial to lateral adhering to the undersurface of the heel, mimicking a stirrup.     

stoma - opening in the abdomen that is created by an operation (ostomy); must be covered at all times by a bag that collects stool.

stomach - main organ of digestion which lies between the esophagus and the small intestine.

stool (Also called feces.) - solid wastes that pass through the rectum as bowel movements. Stools are undigested foods, bacteria, mucus, and dead cells.

streptokinase - a clot-dissolving medication.

stress - mental or physical tension that results from physical, emotional, or chemical causes.

stress fracture - fracture caused by non-traumatic, cumulative overload on a bone; caused by a number of factors including overtraining, incorrect biomechanics, fatigue, hormonal imbalance, poor nutrition, and osteoporosis.

stress incontinence - involuntary leaking of urine which may be caused by coughing or straining.

stress ulcer - upper gastrointestinal ulcer from physical injury such as surgery, major burns, or critical head injury.

striatum - part of the basal ganglia; a large cluster of nerve cells, consisting of the caudate nucleus and the putamen, that controls movement, balance, and walking; the neurons of the striatum require dopamine to function.

stricture (Also called stenosis.) - abnormal narrowing of a body opening. (See also esophageal stricture and pyloric stenosis.)

subchondral tissue - the smooth tissue at the ends of bones, which is covered with another type of tissue (called cartilage).

Sudeck's atrophy - condition characterized by severe pain, swelling, and disability; commonly seen in the foot, but may occur in other joints of the lower limb.

superior vena cava - the large vein that returns blood to the heart from the head and arms.

surgery - operation to remove or repair a part of the body, or to find out if disease is present.

suspensory ligament of lens - a series of fibers that connect the ciliary body of the eye with the lens, holding it in place.

sustention tremor (Also called postural tremor.) - a tremor of a limb that increases when the limb is stretched.

synapse - a tiny gap between the ends of nerve fibers across which nerve impulses pass from one neuron to another; at the synapse, an impulse causes the release of a neurotransmitter, which diffuses across the gap and triggers an electrical impulse in the next neuron.

syncope - light-headedness or fainting caused by insufficient blood supply to the brain.

synovitis - inflammation of the synovial membrane, the tissue that lines and protects the joint.

synovium - a fibrous envelope that produces a fluid that helps to reduce friction and wear in a joint.

systemic treatment (Also called systemic therapy.) - treatment or therapy that reaches and affects cells throughout the body.

systolic blood pressure - the highest pressure to which blood pressure rises with the contraction of the heart.