The Tenant

Trelkovsky (Roman Polanski), a quiet and unassuming man, rents an apartment in Paris whose previous tenant, Egyptologist Simone Choule, attempted to commit suicide by throwing herself out the window and through a pane of glass below. He visits Choule in the hospital but finds her entirely in bandages and unable to talk. Whilst still at Choule's bedside, Trelkovsky meets Simone's friend, Stella (Isabelle Adjani), who has also come to visit. Stella begins talking to Simone, who becomes aware of her visitors. Initially showing some signs of agitation upon seeing them, Choule soon lets out a disturbing cry, then dies. It isn't clear which of the two has caused this reaction. Apparently unaware that Choule is now dead, Trelkovsky tries to comfort Stella but dares not say that he never knew Simone, instead pretending to be another friend. They leave together and go out for a drink and a movie (1973's Enter The Dragon), where they fondle each other. Outside the theatre they part ways.
As Trelkovsky occupies the apartment he is chastised unreasonably by his neighbors and landlord, Monsieur Zy (Melvyn Douglas), for hosting a party with his friends, apparently having a woman over, making too much noise in general, and not joining in on a petition against another neighbor. Trelkovsky attempts to adapt to his situation, but is increasingly disturbed by the apartment and the other tenants. He frequently sees his neighbors standing motionless in the toilet room (which he can see from his own window), and discovers a hole in the wall with a human tooth stashed inside. He receives a visit and a letter from one Georges Badar (Rufus), who secretly loves Simone and has believed her to be alive and well. Trelkovsky updates and comforts the man and spends the night out with him. Gradually he changes his breakfast habits to those of Simone, and shifts from Gauloises to Marlboro cigarettes.
Trelkovsky becomes severely agitated and enraged when his apartment is robbed, while his neighbors and the concierge (Shelley Winters) continue to berate him for making too much noise. He buys a wig and woman's shoes and goes on to dress up (using Simone's dress which he had found in a cupboard) and sit still in his apartment in the dead of night. He suspects that Zy and neighbors are trying to subtly change him into the last tenant, Simone, so that he too will kill himself. He becomes hostile and paranoid in his day-to-day environment (snapping at his friends, slapping a child in a park) and his mental state progressively deteriorates. He has visions of his neighbors playing football with a human head, sees himself staring out of his own window and finds the toilet covered in hieroglyphs. Trelkovsky runs off to Stella for comfort and sleeps over, but in the morning after she has left for work, he concludes that she too is in on his neighbors' plot, and proceeds to wreak havoc in her apartment before departing.
At night he is hit by an elderly couple driving a car. He is not injured too seriously, but receives a sedative injection from the doctor due to his odd behaviorâhe perceives the elderly couple as his landlord Zy and wifeâafter which the couple returns him to his apartment. A deranged Trelkovsky dresses up again as a woman and throws himself out the apartment window in the manner of Simone Choule, before what he believes to be a clapping, cheering audience composed of his neighbors. The suicide attempt, in fact, wakes up his neighbors, who arrive at the scene together with the police just in time for Trelkovsky to crawl up to his apartment and jump one more time.
The end of the movie is enigmatic. Trelkovsky is bandaged up in the same fashion as Simone Choule in the same hospital bed, but we see his and Stella's own visit to Simone. Trelkovsky then lets out the same disturbing cry that Simone had screamed.

Question: What was inside the hole in the wall Trelkovsky discovers?
Answer: human tooth