Reticular formation A network of small groups of neuronal cell bod-
ies scattered among bundles of axons (mixed gray and white mat-
ter) beginning in the medulla oblongata and extending superiorly
through the central part of the brain stem.
Reticulocyte (re-TIK-u-lo-sTt) An immature red blood cell.
Reticulum (re-TIK-u-lum) A network.
Retina (RET-i-na) The deep coat of the posterior portion of the
eyeball consisting of nervous tissue (where the process of vision
begins) and a pigmented layer of epithelial cells that contact the
Retinal (RE-ti-nal) A derivative of vitamin A that functions as the light-
absorbing portion of the photopigment rhodopsin.
Retraction (re-TRAK-shun) The movement of a protracted part of the
body posteriorly on a plane parallel to the ground, as in pulling
the lower jaw back in line with the upper jaw.
Retrograde degeneration (RE-tro-grad de-jen-er-A-shun) Changes
that occur in the proximal portion of a damaged axon only as far
as the first node of Ranvier; similar to changes that occur during
Retroperitoneal (re'-tro-per-i-to-NE-al) External to the peritoneal lin-
ing of the abdominal cavity.
Rh factor An inherited antigen on the surface of red blood cells in
Rh+ individuals; not present in Rh- individuals.
Rhinology (rT-NOL-o-je) The study of the nose and its disorders.
Rhodopsin (ro-DOP-sin) The photopigment in rods of the retina, con-
sisting of a glycoprotein called opsin and a derivative of vitamin A
Ribonucleic acid (RNA) (rT-bo-noo-KLE-ik) A single-stranded nucleic
acid made up of nucleotides, each consisting of a nitrogenous
base (adenine, cytosine, guanine, or uracil), ribose, and a phos-
phate group; three types are messenger RNA (mRNA), transfer
RNA (tRNA), and ribosomal RNA (rRNA), each of which has a
specific role during protein synthesis.
Ribosome (RT-bo-som) An organelle in the cytoplasm of cells, com-
posed of a small subunit and a large subunit that contain ribo-
somal RNA and ribosomal proteins; the site of protein synthesis.
Rigidity (ri-JID-i-te) Hypertonia characterized by increased muscle
tone, but reflexes are not affected.
Rigor mortis State of partial contraction of muscles after death due
to lack of ATP; myosin heads (crossbridges) remain attached to
actin, thus preventing relaxation.
Rod One of two types of photoreceptor in the retina of the eye;
specialized for vision in dim light.
Root canal A narrow extension of the pulp cavity lying within the
root of a tooth.
Rotation (ro-TA-shun) Moving a bone around its own axis, with no
Round window A small opening between the middle and internal
ear, directly inferior to the oval window, covered by the second-
ary tympanic membrane.
Ruffini corpuscle (roo-FE-ne) A sensory receptor embedded deeply
in the dermis and deeper tissues that detects stretching of the
skin. Also called a type II cutaneous mechanoreceptor.
Rugae (ROO-ge) Large folds in the mucosa of an empty hollow organ,
such as the stomach and vagina.
Saccule (SAK-ul) The inferior and smaller of the two chambers in the
membranous labyrinth inside the vestibule of the internal ear
containing a receptor organ for static equilibrium.
Sacral plexus (SA-kral PLEK-sus) A network formed by the ventral
branches of spinal nerves L4 through S3.
Sacral promontory (PROM-on-tor'-e) The superior surface of the
body of the first sacral vertebra that projects anteriorly into the
pelvic cavity; a line from the sacral promontory to the superior
border of the pubic symphysis divides the abdominal and pelvic
Saddle joint A synovial joint in which the articular surface of one
bone is saddle shaped and the articular surface of the other bone
is shaped like the legs of the rider sitting in the saddle, as in the
joint between the trapezium and the metacarpal of the thumb.
Sagittal plane (SAJ-i-tal) A plane that divides the body or organs into
left and right portions. Such a plane may be midsagittal (median),
in which the divisions are equal, or parasagittal, in which the divi-
sions are unequal.
Saliva (sa-LT-va) A clear, alkaline, somewhat viscous secretion pro-
duced mostly by the three pairs of salivary glands; contains vari-
ous salts, mucin, lysozyme, salivary amylase, and lingual lipase
(produced by glands in the tongue).
Salivary amylase (SAL-i-ver-e AM-i-las) An enzyme in saliva that initi-
ates the chemical breakdown of starch.
Salivary gland One of three pairs of glands that lie external to the
mouth and pour their secretory product (saliva) into ducts that
empty into the oral cavity; the parotid, submandibular, and sub-
Salt A substance that, when dissolved in water, ionizes into cations
and anions, neither of which are hydrogen ions (H+) nor hydroxide
Saltatory conduction (sal-ta-TO-re) The propagation of an action
potential (nerve impulse) along the exposed parts of a myelin-
ated axon. The action potential appears at successive nodes of
Ranvier and therefore seems to leap from node to node.
Sarcolemma (sar'-ko-LEM-ma) The cell membrane of a muscle fiber
(cell), especially of a skeletal muscle fiber.
Sarcomere (SAR-ko-mer) A contractile unit in a striated muscle fiber
(cell) extending from one Z disc to the next Z disc.
Sarcoplasm (SAR-ko-plazm) The cytoplasm of a muscle fiber (cell).
Sarcoplasmic reticulum (sar'-ko-PLAZ-mik re-TIK-u-lum) A network of
saccules and tubes surrounding myofibrils of a muscle fiber (cell),
comparable to endoplasmic reticulum; functions to reabsorb
calcium ions during relaxation and to release them to cause
Saturated fat A fatty acid that contains only single bonds (no double
bonds) between its carbon atoms; all carbon atoms are bonded to
the maximum number of hydrogen atoms; prevalent in triglycerides
of animal products such as meat, milk, milk products, and eggs.