When a landlord leases a part of the premises to individual tenants, as in an apartment building, he necessarily retains control over areas used in common and must exercise ordinary care to keep these areas in a reasonably safe condition.' This duty arises because common areas are part of the estate reserved by the landlord for the use and benefit of all the tenants. The responsibility of the lessor extends to the lessee, members of the lessee's family, and all persons on the premises at the invitation of the lessee, whether the invitation be express or implied.