Closeups.

It's a Philodendron. The large "trunk" shows the scars of leaves that have formed and fallen over the years. The smaller ones are offsets which appear to have developed their own root systems and could probably be (gently) separated and moved to pots of their own.
The plant in an outdoors tropical situation would use those roots to scale something - a tree stump or rock, going ever higher seeking light, and use the roots for support as well as nourishment along the way.
A tribute to the tenacity of life, but it would grow better with improved light and fertilizer.

there is a very good houseplant forum ... with some of its own experts ... in regard to further care

i gotta ask.. when in the last 40 years.. have you changed the potting media???

ken

P. bipinnatifidum, tree Philo.

So interesting! You set off a flurry of googling around here. Never suspected that Philodendron was an entire genus. (Rather than just the ubiquitous houseplant-vine that even someone with a brown-thumb can grow.)

I asked again, and yes the container and medium (and window-shade!) are all original mid-70s. (And will broach the topic of increased attention to this "fixture" in the window.)

Thanks very much to all for the education.