What is the flaps of tissue that allow blood to flow in only one direction

A2012 report from the u.s. surgeon general stated: "another common practice is strategically locating tobacco-related marketing materials where young children will be exposed to them. tobacco industry executives acknowledge that products and advertising should be placed at eye level (pollay 2007), but in california, 48% of stores had at least one cigarette marketing item at or below 3 feet from the floor (feighery et al. 2001)." it was also noted that 25% of cigarette displays were next to candy. in addition, a national study found that about one-third of the stores had tobacco ads at low heights. after california banned counter displays, some stores place cigarettes in transparent displays so product brands could still be seen. why did some stores use transparent units to display the cigarettes? to comply with the new rules while still encouraging cigarette purchases via visual exposure to ensure children and minors are not exposed to cigarettes and their eye-catching packaging to minimize exposure to humidity, extending the shelf life of the cigarettes' tobacco to reduce the occurrence of shoplifting, since cigarettes are high-revenue items