Histologists have long ago demonstrated that a muscle is not a simple histological unit, and that it consists of innumerable muscle fibres associated together in fasciculi, each muscle fibre being brought into individual relationship with the central nervous system by its own fibre. Physiologists have been too apt to overlook this fact, and have regarded a muscle as a simple indivisible physiological unit. When a muscle or its nerve are stimulated by an instantaneous electrical shock all the fibres contract together, for they are all stimulated in the same way, and at the same time. This is not the case, according to the investigations of the author, when the muscle contracts reflexly or voluntarily, and especially during any prolonged contraction. The individual fibres are then never completely co-ordinated together, and vary constantly in their “pull.” They are like a body of men pulling on a rope where perfect and prolonged co-ordination is impossible. The proof of this is obtained by the use of very delicate levers attached to different parts (fasciculi) of the same muscle. Two levers attached in this way to a muscle always give slightly different curves, evidence of individuality being very apparent. Experiments were conducted both upon the human masseter muscle and upon the gastrocnemius of a frog, and in each case the curves obtained by the two levers corresponding in the main always differed in detail.