(ă-rak-ĭ-don′ik as′id) A polyunsaturated fatty acid, C20H32O2; a precursor of prostaglandins, essential in nutrition; present abundantly in the amniochorion, decidua, and amniotic fluid.
arachnid
(ă-rak′nid) Any member of the class Arachnida.
Arachnida
(ă-rak′nĭ-dă) A class of arthropods (subphylum Chelicerata) that characteristically have four pairs of legs; includes spiders, scorpions, mites, and ticks.
arachnidism
(ă-rak′nĭ-diz-m) Systemic poisoning following the bite of a spider, especially of the black widow and brown recluse spiders.
arachnodactyly
(ă-rak-no-dak′tĭ-le) Hereditary condition marked by excessive length and slenderness of the bones of the fingers and toes; may be accompanied by relaxed joint ligaments and is usually associated with a connective tissue disorder.
arachnoid
(ă-rak′noid) 1. Having the appearance of a cobweb. 2. The middle of the three membranes covering the brain and spinal cord, between the dura mater and the pia mater;
separated from the pia mater by the subarachnoid space.
arachnoiditis
(ă-rak-noi-di′tis) Inflammation of the arachnoid.
adhesive a. Inflammation of the arachnoid and adjacent pia mater, sometimes causing obliteration of the subarachnoid space.
arachnophobia
(ă-rak-no-fo′be-ă) An inordinate fear of spiders.
arborescent
(ar-bo-res′ent) Treelike; branching.
arborization
(ar-bor-ĭ-za′shun) Denoting the branching of nerve fibers and capillaries.
arborize
(ar′bor-ize) To ramify or branch.
arbovirus
(ar′bo-vi-rus) Any arthropod-borne virus of the genera Alphavirus and Flavivirus (family Togaviridae).
arc
(ark) 1. Anything shaped like an arch or a bow. 2. The luminous line formed by the electric current crossing a gap between two electrodes.
mercury a. An electric discharge through mercury vapor in a vacuum tube, producing ultraviolet rays.
reflex a. The path followed by a nerve impulse in the production of a reflex act.
arch
pharynx (gut) in the mesenchyme of the pharyngeal arches; they are never present all at the same time.
a. of the aorta The curved portion of the aorta between the ascending and descending parts of the thoracic aorta.
costal a. An arch formed by the borders of the inferior aperture of the thorax, comprised of the costal cartilages of ribs seven to ten.
a.’s of foot The two sets of arches (longitudinal and transverse) formed by the bones of the foot.
longitudinal a. The anteroposterior arch of the foot, formed by the seven tarsal and five metatarsal bones and the ligaments binding them together.
palatoglossal a. One of two folds of mucous membrane extending from the posterior edge of the soft palate to the side of the tongue; forms the anterior margin of the tonsillar fossa.
palatopharyngeal a. One of two folds of mucous membrane passing downward from the posterior edge of the soft palate to the lateral wall of the pharynx; forms the posterior margin of the tonsillar fossa.
pharyngeal a. One of a series of five mesodermal arches (bars) in the neck region of the embryo from which several structures of the head and neck develop.
pubic a. Arch on the pelvis formed by the convergence of the inferior rami of the ischium and pubic bones on either side.
superciliary a. An arched prominence above the upper margin of the orbit.
transverse a. The arch of the foot formed by the proximal parts of the metatarsal bones anteriorly and the distal row of the tarsal bones posteriorly.
vertebral a. The arch on the dorsal side of a vertebra which, with the vertebral body, forms the foramen in which the spinal cord is lodged.
zygomatic a. The arch formed by the zygomatic process of the temporal bone and the temporal process of the zygomatic bone.
arctation
(ark-ta′shun) Stricture; narrowing.
arcuate
(ar′ku-āt) Arched.
arcus
(ar′kus), pl. ar′cus Any arch-shaped structure; an arch.
a. juvenilis A gray white ring around the cornea, occurring in the young.
a. senilis An opaque grayish ring around the cornea, occurring in the aged.
area
(ar′e-ă) A distinct part of a surface or space.
aortic a. Area on the chest over the cartilage of the second right rib.
apical a. Area about (a) The tip of the root of a tooth. (b) The apex of a lung. (c) The chest wall corresponding to the apex of the heart (normally the apex beat is in approximately the fifth left intercostal space in the midclavicular line).
auditory a. Region of the cerebral cortex concerned with hearing, occupying the transverse temporal gyri and the superior temporal gyrus.
Broca’s speech a. Area comprising the triangular and opercular portions of the inferior frontal gyrus; it governs the motor aspects of speech.
Brodmann’s a.’s The 47 areas of the cerebral cortex mapped out according to the arrangement of their cellular components.
a. of cardiac dullness Normally a small triangular area on the lower left side of the sternum which, on percussion of the chest, produces a dull sound; it corresponds to the portion of the heart not covered by lung tissue.
a. cribrosa Area of the renal papilla containing 20 or more pores through which the urine oozes into the minor calyces.
frontal a. Portion of the cerebral cortex in front of the central sulcus (fissure of Rolando).
Little’s a. A highly vascular area of the anterior portion of the nasal septum; frequent site of nose-bleed.
macular a. The part of the retina that contains a yellow pigment, is used for central vision, and appears to be free of vessels when viewed with an ophthalmoscope.
mirror a. The reflecting surface of the lens of the eye and the cornea when illuminated with the slit lamp.
mitral a. The chest area over the apex of the heart (approximately the fifth intercostal space in the midclavicular line) where the sound produced by the left atrioventricular (mitral) valve is usually heard most clearly.
motor a. Portion of the cerebral cortex composed of the anterior wall of the central sulcus (fissure of Rolando) and adjacent portions of the precentral gyrus; its stimulation
to the central sulcus (fissure of Rolando); it receives sensory stimuli from the whole body.
premotor a. Area immediately in front of the motor area, concerned with integrated movements.
pulmonic a. Area of the chest at the second left intercostal space where flow sounds across the pulmonary valves are usually heard best.
skip a.’s Areas of the intestinal lining that are relatively uninvolved in the process of Crohn’s disease.
subcallosal a. An area of the cortex in the medial aspect of each cerebral hemisphere, located immediately
in front of the lamina terminalis and caudoventral to the subcallosal gyrus.
tricuspid a. Area of auscultation for murmurs originating from the right atrioventricular valve (tricuspid); the lower left sternal area.
visual a. Area of the occipital lobe of the cerebral cortex concerned with vision;
consists of two parts. Sensory or striate part, occupies the walls of the calcarine sulcus (occasionally extending around the occipital pole onto the lateral surface of the hemisphere); concerned with recognition of size, form, motion, color, illumination, and transparency. Psychic or parastriate part, surrounds the sensory portion; associates visual impressions and past experiences for recognition and identification.
areata
(ar-e-a′tă) Denoting circumscribed areas or patches.
Arenaviridae
(ă-re-nă-vir′ĭ-de) Family of viruses (50 to 300 nm in diameter) that contain single-stranded RNA, multiply in cytoplasm, and appear sandy on electronmicroscopy; includes viruses causing lymphocytic choriomeningitis and Lassa fever.
Arenavirus
(ă-re′nă-vi-rus) Genus of viruses (family Arenaviridae) that includes the Lassa and the lymphocytic choriomeningitis viruses.
areola
(ă-re′o-lă) 1. One of the minute spaces in a tissue. 2. A circular pigmented area around a central point, such as the pigmented area around the nipple on the breast.
argentaffin
(ar-jen′tă-fin) Denoting cells that have an affinity for silver salts.
argentation
(ar-jen-ta′shun) Staining with a silver salt such as silver nitrate.
argentous
(ar-jen′tus) Relating to silver, denoting a compound containing silver in its lower valence.
arginine
(ar′jĭ-nēn) (Arg) An essential amino acid, C6H14N4O2, derived from the hydrolysis of protein.
argon
(ar′gon) A colorless, odorless, gaseous element constituting about 1% of the earth’s atmosphere; symbol Ar, atomic number 18, atomic weight 39.6.
argyria
(ar-jir′e-ă) Chronic silver poisoning causing a permanent grayish discoloration of the skin, conjunctiva, cornea, and internal organs due to the prolonged use of preparations containing silver compounds.
arm
(arm) The upper limb of the human body, especially between the shoulder and the elbow.
nuchal a. A fetal arm that is positioned around the back of the neck; sometimes seen in breech deliveries.