The muscular tissue of the heart is not under the control of the will; it, however, is cross-striped, and more like the voluntary than the ordinary involuntary muscle, though it differs in some respects from both.

Speaking generally, we may say that the movements necessary for the nutrition of the body are not left for us to look after ourselves, but are carried on by muscles which work involuntarily ; the blood is pumped round by the heart, and food churned up in the stomach and passed along the intestines, whether we think about it or not.

What is their function? What are they composed of ? What is seen when a cell from an involuntary muscle is examined with the microscope?

Is the heart muscle voluntary? In what respect does it resemble voluntary muscle?

What movements of the body does nature not leave to our own control? Give examples.

The Chemical Composition Of Muscle

Muscle contains about 75 per cent, of water; and a considerable quantity of salines. Living, resting muscle is alkaline to test paper; hard-worked or dying muscle is acid. Its chief organic constituents are proteid or albuminous substances (p. 21), and of these the most abundant in a perfectly fresh muscle is myosin. Soon after death the myosin clots. Dilute acids dissolve myosin and turn it into syntonin, which used to be thought the chief proteid of muscle.