Nowadays, says the narrator, toilets are designed to look nice in bathrooms. But in the engineer's old flat, this was not the case. The toilet did not disguise the fact that it was basically the open end of a sewer pipe.
Tereza goes into the engineer's bathroom and suddenly desires to void her bowels, because it would mean utter humiliation, the extreme of being only a body and not a soul. Her soul is no longer looking on, interested, at her body. Instead, it has retreated deep inside her body, "waiting desperately for someone to call it out" (4.18.4).