This is analogous to the fuse box in your house, distributing electricity from the main electric cable to all of your individual appliances/outlets):
Or a highway interchange which can route cars off an interstate to multiple local roads:
This is a real human brachial plexus:
However, in clinical practice, we are usually dealing with closed (traction) injuries to the plexus, so we are not too concerned about actual anatomy.
We are more interested in figuring out where the lesion lies within the plexus based on the patient’s clinical deficit.
We are therefore more likely to use diagrams of the plexus which distort anatomy, but emphasize branches and intersections, to localize these lesions:
For example, a patient with weakness of deltoid (axillary nerve) and triceps (radial nerve) but not biceps (musculocutaneous nerve) must have a lesion in the posterior cord (green in the figure above).
Earlier maps were geographically correct, but as more lines and stations were added these maps became more cumbersome and confusing.
Beck realized that because the railways were underground, the physical locations of the stations were irrelevant to the traveller wanting to know how to get to one station from another, so he devised a simplified map showing only stations, straight line segments connecting them, and the river. All lines ran only vertically, horizontally, or on 45 degree diagonals, ignoring the actual geography:
You can easily use this kind of map to find the best way to get from Park Royal to Oxford Circus, with the least number of stops and line changes.
Do you see the similarity?
Other examples of medical uses of Beck’s London Underground map: