Long ago the muscular and nervous structures of the body, because of their dominant functional importance, were classed together as the "master tissues." The activity of the muscles is known to be dependent on metabolic changes in which the production of acid, notably carbonic acid, and the consumption of oxygen are the main chemical characteristics. Incident to these is the measurable production of work and heat. Whether nervous tissue behaves in an analogous manner has not been so clearly determined. For the nerve cells, as they occur aggregated in the brain and various ganglionic masses, the probability of an active chemical exchange incident to their function has generally been accepted. They show symptoms of fatigue and a sensitiveness to lack of oxygen, somewhat as a muscle does.
For the nerve fibers, however, the evidence has been far less cogent. Before Helmholtz, in 1850, measured the rate of transmission of the