A circuit of mutually exciting neurones capable of approaching and maintaining a state of permanent excitation has been described by Rashevsky as a possible mechanism for the production of conditioned reflexes. The mode of approach of such a circuit to the steady state is examined in some detail under slightly more general assumptions, and an estimate is made of the order of magnitude of the time required to reach this state.

A Bi-factor analysis is made of Professor Thurstone's battery of fifty-seven tests employing his tetrachoric correlations. Although this analysis is made entirely independent of his multiple factor analysis, a very close agreement is found between the group factors obtained here and Thurstone's verbal descriptions previously published.

The properties of a simple neural mechanism are developed theoretically and interpreted in terms of observable variables. The results are discussed in connection with experimental data in cases for which the latter are available.

In previous formulations the assumption has been made that the intensity of excitation or inhibition varies with distance. In this paper the simple case of a series of identical neurones is treated. The development shows five possible relations. In future theoretical formulations it is no longer necessary to postulate arbitrary relations in case any of the derived forms can be used.