Isthmus

(is'mus) [G. isthmos.'] i. constriction connecting two larger parts of an organ or other anatomical structure. 2. narrow pa age connecting two larger cavities. Guyon's i., i, uteri, Haller's i., Haller's* fretum. His's i., i. rhombencephali, i. aor'tse [BNA], a constriction of the arch of the aorta just beyond the origin of the subclavian artery, i. cartilag'in is au'ris [BNA], a narrow bridge connecting the cartilage of the external auditory meatus and the lamina tragic a with the main portion of the cartilage of the auricle, i. fau'cium [BNA], fauces, i. glan'dulse thyreoid'ea [BNA], of the thyroid, the central part of the thyroid gland joining the two lateral lobes, i. gy'ri fornica'ti [BNA], i, of the fornicate, gyrus, i. of the limbic lobe, the narrow posterior portion of the gyrus cinguli, behind and below the splenium of the corpus caUosum, which joins the hippocampal gjfrus. i. mea'tus acus'tic i ezter'ni, isthmus of the external auditory meatus, the narrowest portion of this canal near its inner termination, i. pharyngonasa'lis, pharyngeal i., the aperture between the oral pharynx and the nasopharynx, bounded by the posterior palatine arches and the soft palate, i. prost a'tae [BNA], the anterior portion of the base of the -prostate gland, i. Thombenceph'ali [BNA], rhomb encephalic i.; (i) a constriction in the embryonic neural canal separating the mesencephalon from the rhombencephalon, or metencephalon; (2) the anterior portion of the rhombencephalon connecting with the mesencephalon; it consists of the brachia conjunctiva, the trigone of the lemniscus, the anterior medullary velum, and the anterior portion of the rhomboidal fo a, i. tu'bee audit i'TS [BNA], isthmus of the auditory (Eustachian) tube, the narrowest portion of the Eustachian tube at the junction of the cartilaginous and bony portions, i. tu'bse uteri'nte [BNA], the narrow portion of the Fallopian tube adjoining the u'terus. i. u'teri, Guyon's i„ an elongated