This section is from the book "Human Physiology For The Use Of Elementary Schools", by Charles Alfred Lee. Also available from Amazon: Human Physiology, for the Use of Elementary Schools.
1. Man is distinguished from the vegetable world, not only by his possessing a nervous system, organs of sense, and voice, but also by the power of locomotion, or moving from place to place. This power connects him with the external world, enlarges his sphere of action and increases his means of acquiring knowledge. It implies that he has a will, and that these motive organs are under the influence of volition; else he would be the sport of ehance and wander about without a motive.
2. Many of the functions of the body are not under the control of the will; such are digestion, absorption, circulation, respiration, and secretion, which go on as well when we are asleep as when awake. They are possessed, at least some of them, by vegetables as well as animals, and are therefore called organic functions.
3. The agents of locomotion are the bones and muscles; but they would be useless for motion were they not supplied with nerves of voluntary motion, and thus brought under the influence of the will. The bones are tied together by means of strong fibrous ligaments or cords, allowing the joints great freedom and extent of motion, as we see in the shoulder and hip joints.
4. We then have the bones, which act as levers ; the muscles are the moving power, and the brain and nerves are the vital agents, which set the machinery in motion. Muscles alone have the power of contraction, and it is one of the most remarkable properties of life. Were it not for this, the food could not be digested, the blood could not be circulated, and the iris could not guard the eye against the admission of too much light, which would speedily destroy the vision.
5. The shape of muscles is various, some are round, some flat, and the fibres of which they are composed, are connected by means of cellular membrane. Some are penni form, or made of bundles of fibres, diverging from a central line, like the feathers of a quill. Muscles compose a large part of the bulk of the body ; and when they contract, the fibres shorten and become harder, as may easily be perceived by placing one hand on the middle of the arm and bending the elbow, or on the temple and closing firmly the lower jaw.
6. The force with which a muscle contracts, depends on the physical condition of the muscle and the energy of the brain. When the fibres of the muscle are large and firm, they will contract with more force than when they are small, soft, and delicate. We see some persons, who labour under great mental excitement, perform astonishing feats of strength, although, perhaps, they may not have muscles of the ordinary size.
7. The knee pan has often been split in two by the contraction of the muscles of the leg; a horse has been known to break its under jaw by biting a piece of iron. Men have been known to lift eight and nine hundred pounds weight; to break ropes two inches in circumference, and to bend a round piece of iron, a yard long, and three inches in circum ference, to a right angle, by striking it across the left arm, between the shoulder and the wrist, with the right hand.*
8. The force of muscular contraction is greatly increased by exercise. The strength of an active man labouring to the greatest possible advantage, is estimated to be sufficient to raise ten pounds, ten feet in a second, for ten hours in a day ; or to raise one hundred pounds, one foot in a second, or thirty six thousand feet in a day ; or three millions, six hundred thousand pounds, or four hundred and thirty two thousand gallons, one foot in a day. The weakest men in health can generally lift about one hundred and twenty five pounds, and the strongest of ordinary men four hundred pounds. The daily work of a horse is equal to that of five of six men.
* See Dunglison's Physiology.
9. There is much difference in the velocity of muscular contraction, as it is regulated entirely by the will. The swiftest race horse on record was capable of going a mile in a minute ; yet this is trifling compared with the velocity of birds, or even of many small insects. It has been ascertained that a pigeon hawk can fly one hundred miles in an hour ; the eider duck ninety miles an hour, and the common crow, twenty five miles an hour. The swallow flies ninety two miles an hour, and the swift is said to fly two hundred and fifty miles in the same space of time.
10. A falcon belonging to Henry IV. of France, escaped from Fontainbleau, and in twenty four hours after was in Malta, a distance of one thousand three hundred and fifty miles, making a velocity of fifty seven miles an hour, supposing him to have been on the wing all the time; but as such birds do not fly by night, his flight was probably at the rate of seventy five miles per hour. This will give us some idea of the wonderful velocity of the contractions of the muscles of the wings of birds of rapid flight.
11. But this rapidity of motion is much under the influence of habit. How awkward are the first attempts at writing, drawing, dancing, or playing on musical instruments, and with what ease and grace are they performed after a little practice. The same is true with regard to public speaking, or any thing which requires voluntary motion.
12. It is a law of the human system, that relaxation must follow contraction,-or rest, exercise; and although the duration of action of the voluntary muscles is in a great degree under the control of the will, yet it cannot be continued long. This duration will be shorter in proportion as the contraction is violent or moderate. All muscles do not act at the same time, for as some are contracting, such as the flexors, or those that bend the limbs, the extensors, or those that straighten them, are relaxed.
13. It is by the constant action of the muscles, that the body is kept in an erect position. If a person gets asleep while sitting or standing up, the head falls forwards, and if he did not wake, the body would fall likewise. The same happens when a person is deprived of sense, by a fit of apoplexy or palsy, or by the intemperate use of ardent spirits.