Sinoatrial (SA) node (si-no-A-tre-al) A small mass of cardiac muscle
fibers (cells) located in the right atrium inferior to the opening of
the superior vena cava that spontaneously depolarize and gener-
ate a cardiac action potential about 100 times per minute. Also
called the pacemaker.
Sinus (ST-nus) A hollow in a bone (paranasal sinus) or other tissue;
a channel for blood (vascular sinus); any cavity having a narrow
Sinusoid (SF-nu-soyd) A large, thin-walled, and leaky type of capillary,
having large intercellular clefts that may allow proteins and blood
cells to pass from a tissue into the bloodstream; present in the liver,
spleen, anterior pituitary, parathyroid glands, and red bone marrow.
Skeletal muscle An organ specialized for contraction, composed
of striated muscle fibers (cells), supported by connective tissue,
attached to a bone by a tendon or an aponeurosis, and stimu-
lated by somatic motor neurons.
Skin The external covering of the body that consists of a superficial,
thinner epidermis (epithelial tissue) and a deep, thicker dermis
(connective tissue) that is anchored to the subcutaneous layer.
Skull The skeleton of the head consisting of the cranial and facial
Sleep A state of partial unconsciousness from which a person can
be aroused; associated with a low level of activity in the reticular
Sliding-filament mechanism The explanation of how thick and thin
filaments slide relative to one another during striated muscle
contraction to decrease sarcomere length.
Small intestine A long tube of the gastrointestinal tract that begins
at the pyloric sphincter of the stomach, coils through the central
and inferior part of the abdominal cavity, and ends at the large
intestine; divided into three segments: duodenum, jejunum, and
Smooth muscle A tissue specialized for contraction, composed of
smooth muscle fibers (cells), located in the walls of hollow inter-
nal organs, except for the heart, and innervated by autonomic
Sodium-potassium pump An active transport pump located in the
plasma membrane that transports sodium ions out of the cell
and potassium ions into the cell at the expense of cellular ATP. It
functions to keep the ionic concentrations of these ions at physi-
Soft palate (PAL-at) The posterior portion of the roof of the mouth,
extending from the palatine bones to the uvula. It is a muscular
partition lined with mucous membrane.
Solution A homogeneous molecular or ionic dispersion of one or
more substances (solutes) in a dissolving medium (solvent) that is
Somatic cell division (so-MAT-ik) Type of cell division in which a single
starting parent cell duplicates itself to produce two identical cells;
consists of mitosis and cytokinesis.
Somatic nervous system (SNS) The portion of the peripheral nervous
system consisting of somatic sensory (afferent) neurons and
somatic motor (efferent) neurons.
Spasm (SPAZM) A sudden, involuntary contraction of skeletal
Spasticity (spas-TIS-i-te) Hypertonia characterized by increased
muscle tone, increased tendon reflexes, and pathological reflexes
Spermatic cord (sper-MAT-ik) A supporting structure of the male
reproductive system, extending from a testis to the deep inguinal
ring, that includes the ductus (vas) deferens, arteries, veins, lym-
phatic vessels, nerves, cremaster muscle, and connective tissue.
Spermatogenesis (sper'-ma-to-JEN-e-sis) The formation and develop-
ment of sperm in the seminiferous tubules of the testes.
Sperm cell A mature male gamete. Also termed a spermatozoon
Spermiogenesis (sper'-me-o-JEN-e-sis) The maturation of spermatids
Sphincter (SFINGK-ter) A circular muscle that constricts an opening.
Sphygmomanometer (sfig'-mo-ma-NOM-e-ter) An instrument for
measuring arterial blood pressure.
Spinal cord (SPT-nal) A mass of nerve tissue located in the vertebral
cavity from which 31 pairs of spinal nerves originate.
Spinal nerve One of the 31 pairs of nerves that originate on the
spinal cord from posterior and anterior roots.
Spinal shock A period from several days to several weeks following
transection of the spinal cord and characterized by the abolition
of all reflex activity.
Spinothalamic tracts (spT-no-tha-LAM-ik) Sensory (ascending) tracts
that convey information up the spinal cord to the thalamus for
sensations of pain, temperature, itch, and tickle.
Spiral organ The organ of hearing, consisting of supporting cells and
hair cells that rest on the basilar membrane and extend into the
endolymph of the cochlear duct. Also called the organ of Corti
Spirometer (spF-ROM-e-ter) An apparatus used to measure lung
volumes and capacities.
Spleen (SPLEN) Large mass of lymphatic tissue between the fundus
of the stomach and the diaphragm that functions in formation
of blood cells during early fetal development, phagocytosis of
ruptured blood cells, and proliferation of B cells during immune
Spongy (cancellous) bone tissue Bone tissue that consists of an
irregular latticework of thin plates of bone called trabeculae;
spaces between trabeculae of some bones are filled with red
bone marrow; found inside short, flat, and irregular bones and in
the epiphyses (ends) of long bones.
Sprain Forcible wrenching or twisting of a joint with partial rupture
or other injury to its attachments without dislocation.
Squamous (SKWA-mus) Flat or scalelike.
Starvation (star-VA-shun) The loss of energy stores in the form of
glycogen, triglycerides, and proteins due to inadequate intake
of nutrients or inability to digest, absorb, or metabolize ingested
Stasis (STA-sis) Stagnation or halt of normal flow of fluids, as blood
or urine, or of the intestinal contents.