In the old days, before laptops, we used to call the case and everything in it the CPU. You had your monitor, keyboard, mouse, maybe printer, maybe modem, and they would all plug into the “CPU.” Yes, we knew there was also a chip inside called that but we didn’t get all pedantic about it.
With that in mind: Place the CPU fan on the heat sink… That’s exactly what they did.
Oh, yes, I forgot it was also the hard drive! Now that I think about it, I’m not sure the proper name. Today I’d just call it the tower, but I’m not sure what the square ones that sat with the monitor on top would be called.