other to the inner part of the lower jaw, which, when it contracts, draws the whole tongue forwards, causing a part of it to protrude; the eyelids and lips are closed by circular muscles surrounding them, which, by contraction, draw together those parts. [Illustration: FIG. 77.--FIBRILS OF VOLUNTARY MUSCLE, MAGNIFIED.] [Illustration: FIG. 78.--THE SAME, MORE HIGHLY MAGNIFIED.] The muscles form the flesh of an animal, or that part which is of a red colour, and which is called lean (in contra-distinction to fat), they are composed of fibres, each fibre made up of a number of fibrils, and the whole bound up together by means of a fine membrane, called areolar tissue. Each of these fibrils consists of a number of cells pressed closely together and having a peculiar bearded appearance (figs. 77, 78). It is by the sudden approximation and flattening of these cells that the muscle is shortened during contraction, but the contraction of each fibril is only for an instant, and then a relaxation and elongation of the cells takes place, which is followed by a second contraction, and so on. Now, the way in which a muscle keeps in a state of permanent contraction is this: the various fibrils or strings of cells relieve each other instead of all contracting and all relaxing together, and in this way a part only of the fibrils are in a state of contraction at any given time, while the others are relaxed, and as one begins to relax another contracts, and so keeps up a state of contraction in the muscle up to a certain point, when the contractions become gradually more feeble, and finally cease altogether; the muscle is then exhausted, but, if allowed to rest awhile, it again obtains its contractile power. This peculiar contractile power is a physical one, and exists for a short time after death, that is, until its perfect structure is altered by the beginning of decomposition; and what is a curious fact, the muscles of a dead body stimulated to contraction by pricking or galvanism, will tire and again obtain their contractility by rest, just as in the living body. The nature of the stimulus which causes the muscles to contract during life is not known; but, whatever it may be, it is conveyed from the spinal cord and brain, through the medium of a set of nerves, called the "motor nerves," or nerves of motion, to the muscles, to every fibre of which they are distributed. There is, however, another set of nerves which have nothing to do with motion, although exactly similar in appearance and generally associated and bound up in the same sheath with them; these are the nerves of sensation. They take their origin from every part possessed of sensibility or feeling--more especially the skin--and convey every impression of feeling to the brain. The muscles are not all under the control of the will, for the muscles of the heart, bowels, &c. are quite out of our control. Those which are subservient to our will are