Body Part Classifier (BPCL) is a type of classifier that refers to a part of the body beyond the frame of the signing areas. E.g. legs, back, feet, etc.
For example, you utter the ASL word #foot and then use its classifier (e.g. the passive hand) to represent the foot.
For a brain or a heart, you use the "claw-5" classifier handshape of both hands in the mid-air space.
Understand the difference between BPCL and BCL (body classifier).
Also see Describing a female reproductive organ, using BPCL.