| AR | absolute risk; accounts receivable; achievement ratio; actinic reticuloid [syndrome]; active resista... |
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| ARA | Academy of Rehabilitative Audiometry; acetylene reduction activity; American Rheumatism Association;... |
| ARR | aortic root replacement |
| AWAR | anterior wall of aortic root |
| BOR | basal optic root; before time of operation; bowels open regularly; branchio-oto-renal [syndrome] |
| Huxley's sheath | A layer of cells interposed between Henle's layer and the cuticle of the inner root sheath of the hair follicle. Synonym: Huxley's membrane, Huxley's sheath. (05 Mar 2000) |
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| sheath | A protective covering. The lower part of leaf enveloping stem or culm. A secreted, tubular structure formed around a chain of cells or around a bundle of filaments, cells within a sheath may or may not subsequently separate from the sheath. A layer of outer membrane covering the bacterial flagella. (09 Oct 1997) |
| sheath ligaments | See: fibrous digital sheaths of hand, fibrous digital sheaths of foot, fibrous tendon sheath. (05 Mar 2000) |
| sheath of eyeball | A condensation of connective tissue on the outer aspect of the sclera from which it is separated by a narrow cleftlike episcleral space; the sheath is attached to the sclera near the sclerocorneal junction and blends with the fascia of the extraocular muscles. Synonym: vagina bulbi, capsula bulbi, eye capsule, fascia bulbi, sheath of eyeball, Tenon's capsule, vagina oculi. (05 Mar 2000) |
| sheath of Key and Retzius | <anatomy> The delicate bands of connective tissue among nerve fibres. Origin: NL, fr. Gr. Within + a sinew, nerve. Source: Websters Dictionary (01 Mar 1998) |
| sheath of Schwann | <anatomy> The delicate outer sheath of a nerve fibre; the primitive sheath. The perineurium. Origin: NL, from gr. Nerve + peel, skin. Source: Websters Dictionary (01 Mar 1998) |
| sheath of Schweigger-Seidel | <geometry> A solid, all plane sections of which are ellipses or circles. See Conoid, 2 . The ellipsoid has three principal plane sections, a, b, and c, each at right angles to the other two, and each dividing the solid into two equal and symmetrical parts. The lines of meeting of these principal sections are the axes, or principal diameters of the ellipsoid. The point where the three planes meet is the center. Ellipsoid of revolution, a spheroid; a solid figure generated by the revolution of an ellipse about one of its axes. It is called a prolate spheroid, or prolatum, when the ellipse is revolved about the major axis, and an oblate spheroid, or oblatum, when it is revolved about the minor axis. Origin: Ellipse: cf. F. Ellipsoide. Source: Websters Dictionary (01 Mar 1998) |
| sheath of styloid process | A crest of bone (edge of the tympanic portion of the temporal bone) running from the front and medial side of the mastoid process to the spine of the sphenoid; it splits to ensheath the base of the styloid process. Synonym: vagina processus styloidei, vaginal process. (05 Mar 2000) |
| sheath of thyroid gland | Covering of the thyroid gland external to its capsule formed by a splitting of the pretracheal layer of deep cervical fascia at the gland's posterior border; the anterior lamina covers the gland anterolaterally, attaching to the arch of the cricoid cartilage superior to the isthmus of the gland (causing it to move with the trachea during elevation/depression of the larynx); the posterior lamina passes posterior to the oesophagus to blend with the buccopharyngeal fascia; inferiorly, the sheath extends along the inferior thyroid veins to open into the superior mediastinum (hence, expansion of the thyroid, as by goiter, can take this direction). (05 Mar 2000) |
| sheath process of sphenoid bone | A thin lamina of bone that extends medially under the body of the sphenoid bone from the medial lamina of the pterygoid process; it articulates with the vomer and the palatine bone. Synonym: processus vaginalis ossis sphenoidalis, sheath process of sphenoid bone. (05 Mar 2000) |
| sheath rot | ulcerative posthitis |
| sheath-winged | <zoology> Having elytra, or wing cases, as a beetle. Source: Websters Dictionary (01 Mar 1998) |
| nerve sheath tumours | Tumours arising in the sheath surrounding the axon of some nerve cells and formed by the plasma membrane of schwann cells in the peripheral nervous sytem and by oligodendrocytes in the central nervous system. Neurofibromas and neurilemmomas are the most commonly occurring tumours of this type. (12 Dec 1998) |
| Neumann's sheath | A layer of tissue relatively resistant to the action of acids, which forms the walls of the dentinal tubules. Synonym: Neumann's sheath. (05 Mar 2000) |
| neurovascular sheath | Fibrous tissue enveloping and binding together arteries, their accompanying veins (venae comitantes) and nerves which run together; often it is merely the adventitious tissue of the neurovascular structures, but may be highly developed as a distinct fascial layer (e.g., in the case of the carotid or axillary sheaths). (05 Mar 2000) |