In biology a labrum can refer to one of three things: part of the mouthparts of an arthropod, a cartilagenous component of the shoulder or a similar segment of cartilage in the hip. The shoulder and hip labra are the glenoid and acetabular labra, respectively.
Most arthropods, except for sea spiders, have a small flap in front of the mouth. This is the labrum. A similar structure on the underside of the mouth is the labium. These are the upper and lower lips of arthropods. An arthropod labrum has muscles and nerves similar to an appendage, and many entomologists believe that it did indeed originate from an appendage but is now reduced through evolution.
In human anatomy, labra are rings of cartilage that line and deepen the ball-and-socket joints of the shoulder and hip. The glenoid, or shoulder, labrum acts like a washer to lend stability to the joining of the humerus with the glenoid cavity. The acetabular, or hip, labrum has a similar function in preventing the femur from slipping out of its socket. Both labra are prone to tearing if they are over-stressed. A common cause of tearing of the acetabular labrum is the pivoting motions of ballet dancers. Stretching before strenuous activity helps to prevent tearing of these cartilages.