In the metaphorical sense, the shell might be a "carry-on" bag on wheels, stuffed full of cables, discs, dongles, batteries, plugs and paper documents (just in case of disc failure).
I should promise that I use this term in a large and metaphorical sense including dependence of one being on another.
This is true in both a literal and a metaphorical sense: when nervous or stressed we're less likely to notice ideas that are at the edge of our consciousness.
I do not, as is often done, use the word in any vague, uncertain, approximate, or metaphorical sense.
