• (a.) Extending across an isthmus, as at Suez or Panama.
Across any isthmus; specifically, across the isthmus of the fornicate gyrus, denoting the gyrus transitivus. ... (05 Mar 2000) ... 
Type: Term Pronunciation: trans-is′mē-ăn Definitions: 1. Across any isthmus; specifically, across the isthmus of the fornicate gyrus, denoting the gyrus transitivus.
transisthmian Across any isthmus; specifically, across the isthmus of the fornicate gyrus (horseshoe-shaped cortical convolution bordering the hilus of the cerebral hemisphere), denoting the gyrus transitivus (prominent rounded elevations that form the cerebral hemispheres).