A description of the low- and high-temperature behavior of liquid He is given in terms of an extended Matsubara-Matsuda lattice model. Paying particular attention to the kinematical interaction and to the hard core of the bosons leads to a unified theory for the low-temperature behavior and for the lambda transition. Since the hard-core boson operators in the lattice model correspond to spin one-half operators, one can interpret the hamiltonian as the hamiltonian of a magnetic system of oblate form, i.e., the x-y coupling is stronger than the z coupling. A rotation introduced in the space of these operators results in a hamiltonian consisting of three parts: one part is of the anisotropic Heisenberg form, the second contains pairs of creation-annihilation operators at different lattice sites and the third part consists of an interaction between creation-annihilation operators and the z component of the spin operators. A particular choice of the rotation angle eliminates the third part.
