Transmission housing (1) having a body (2) formed by two shells (2A, 2B) and comprising a rotary input shaft (4) and an output shaft (5), the rotary input shaft (4) comprising a driving end (6) and a threaded portion (7) and having mounted on it a rolling-element bearing (8) arranged between the threaded portion (7) and the driving end (6), said housing (1) creating, inside the enclosure (3), a station (12) for receiving the rolling-element bearing (8) and a station (13) for receiving the threaded portion (7). The housing (1) comprises a barrier (14) positioned between said stations (12, 13), this barrier (14) extending around said input shaft (4), creating a through-passage (15) for access for the input shaft (4) from one station (12) to the other (13), this barrier (14) being formed in at least two portions (16, 17), each barrier portion (16, 17) being produced in a single piece with the associated shell (2A, 2B), at least one of the barrier portions (16, 17) being shaped so as to create at least one deflector (161) formed at least partially by an inclined surface having a slope that descends from the body (2) of the housing (1) towards the input shaft (4), when the input shaft (4) is positioned vertically with the rolling-element bearing (8) extending above the threaded portion (7) of the input shaft (4).