ABSTRACT
The preceding chapters have introduced three distinct kinds of functional units—reflexes, oscillators, and servomechanisms. These are some of the building blocks of coordinated action. The higher levels of the nervous system construct complex action sequences (e.g., the building of a nest or a search for food) by allowing appropriate combinations of functional units to operate either simultaneously or in suitable sequence. Reflexes, oscillators, and servomechanisms do not exhaust the major categories of functional units. There remain, no doubt, basic units of action whose essential features have not yet been given a clear experimental characterization. But the three categories that have been experimentally documented provide a foundation for considering principles of hierarchical organization.
