In a mechanical gear transmission, a leaked lubricant liquid, used to lubricate an external ring gear meshing with at least one internal gear wheel, is collected and reintroduced in the system by means of a manifold having a first concave portion, which has a concavity facing towards the external ring gear, so as to house a mass of liquid moved outwards as a result of a centrifugal effect, and at least a second concave portion, which is designed to receive the leaked liquid and has a concavity facing towards the first concave portion; channelling walls being provided to channel the leaked liquid towards one or the other concave portion.