This shows that while the third or lowest level does its own work, it is yet in a sense under the weight--what physiologists call the inhibiting action--of the higher brain masses.

It is not allowed to magnify its part too much, nor to work out of its proper time and measure.

The nervous apparatus involved in these "third-level" functions may be called the "reflex circuit" (see Fig.

2), the path being from the sense organ up to the centre by a "sensory" nerve, and then out by a "motor" nerve to the muscle.
[Illustration: FIG.

2 .-- _s c mt_ = reflex circuit; _s c sp mp c mt_ = voluntary circuit.] Going upward in the nervous system, we next find a certain group of bodies within the gross mass of the brain, certain centres lying between the hemispheres above and the medulla and spinal cord below, and in direct connection by nervous tracts with both of these.