tunica
noun
tu·ni·ca ˈtü-ni-kə 
ˈtyü-
plural tunicae ˈtü-nə-ˌkē
-ˌkī,
-ˌsē,
ˈtyü-
: an enveloping membrane or layer of body tissue
Example Sentences
Recent Examples on the Web But when too much pressure is placed on the tissue, the external envelope surrounding the corpus cavernosa -- called the tunica albuginea -- can tear, causing a corporal rupture, according to Purohit. 
Mark Lieber, CNN, 18 Apr. 2018
Mark Lieber, CNN, 18 Apr. 2018 Word History
Etymology
Latin, tunic, membrane
First Known Use
circa 1698, in the meaning defined above
Medical Definition
tunica
noun
tu·ni·ca ˈt(y)ü-ni-kə
plural tunicae -nə-ˌkē -ˌkī -ˌsē
-ˌkī -ˌsē : an enveloping membrane or layer of body tissue