- 1. An intersection or crossing of two lines. 2. [TA] In anatomy, a decussation or crossing of two fibrous bundles, such as tendins, nerves, or tracts. 3. In cytogenetics, the site at which two homologous chromosomes make contact (thus appearing to be crossed), enabling the exchange of genetic material during the prophase stage of meiosis. SYN: chiasma [TA]. [G. chiasma]- optic c. [TA] a flattened quadrangular body in front of the tuber cinereum and infundibulum, the point of crossing or decussation of the axons of the optic nerves; axons from the nasal retina cross to the opposite side while axons from the temporal retina run directly caudal without crossing, some pass transversely on the posterior surface between the two optic tracts and others pass transversely on the anterior surface between the two optic nerves. SYN: chiasma opticum [TA], optic decussation.- tendinous c. of the digital tendons [TA] crossing of the tendons, the passage of the tendons of the flexor digitorum profundus (flexor digitorum longus in the foot) through the interval left by the decussation of the fibers of the tendons of the flexor digitorum superficialis (flexor digitorum brevis in the foot). SYN: chiasma tendinum [TA], Camper c..
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* * *chi·asm (kiґaz-əm) [L., Gr. chiasma] a decussation or X-shaped crossing; see chiasma.
Medical dictionary. 2011.