tunica vaginalis

noun

tunica va·​gi·​na·​lis -ˌvaj-ə-ˈnā-ləs  -ˈnal-əs 

plural tunicae vaginales -ˈnā-(ˌ)lez  -ˈnal-(ˌ)ēz 

: a pouch of serous membrane covering the testis and derived from the peritoneum