A blowscreen was a component of Federation refitted Constitution-class starships in the 23rd century found within Jefferies tubes. Aboard the USS Enterprise-A in 2287, Montgomery Scott directed James T. Kirk, Spock, and Leonard McCoy to take a right at the hydro vent and then left at the blowscreen to reach a turboshaft that led to the forward observation room. (Star Trek V: The Final Frontier)