If your have good eyesight from the distance that you're at you can see the vessels leading out of the heart and into the lungs, and the lungs are these darker spaces within the ribcage.

如果你的视力足够好的话,你们能看到,由心脏发出血管,进入到肺部,肺是胸腔里这片比较暗的部分

耶鲁公开课 - 生物医学工程探索课程节选