165. The length of time which a muscle may remain contracted, varies. The duration of the contraction of the voluntary muscles, in some measure, is in an inverse ratio to its force. If a muscle has contracted with violence, as when great effort is made to raise a heavy weight, relaxation will follow sooner than when the contraction has been less powerful, as in raising light bodies.
166. The velocity of the muscular contraction depends on the will. Many of the voluntary muscles in man contract with great rapidity, so that he is enabled to utter distinctly 79 fifteen hundred letters in a minute; the pronunciation of each letter requiring both relaxation and contraction of the same muscle, thus making three thousand actions in one minute. But the contraction of the muscles of some of the inferior animals surpasses in rapidity those of man. The race-horse, it is said, has run a mile in a minute; and many birds of prey will probably pass not less than a thousand miles daily.
Give experiment 2d. 164. With what is each muscular fibre supplied? What effect has this stimulus on the muscles? 165. how long does a voluntary muscle remain contracted? 166. On what is the velocity of muscular contraction dependent? How many letters may be pronounced in a minute?
167. The functions of the involuntary muscles are necessary the digestion of food, the absorption and circulation of the nutritive fluids. They could not be trusted with safety to the control of the will, lest the passions or the indiscretions of the person should continually avert those operations so necessary to health, and even to life. The Divine Builder of this complicated machine has wisely ordered that the muscles upon which these motions depend, shall act under the impression of their proper stimulants, without the control of the individual.
168. Again, there are certain operations which could not be safely intrusted to the absolute government of the voluntary muscles, or entirely removed from their control. Thus life can be supported only a few minutes without breathing; but it would be impossible to perform the daily vocations of life if we were compelled to breathe at all times, or at perfectly regular intervals.
169. It has been observed that, among men of the same size, a wide difference exists in their strength and activity—qualities which depend upon the size and number of the nerves, the size and activity of the brain, and the education, or training of the muscles. Men having large nerves leading 80 to the muscles, with the brain active, and muscles well trained will perform feats of strength and agility, that other men, of the same size, cannot effect. Rope-dancers, harlequins, and other performers of feats, are persons thus constituted.
How many contractions and relaxations of the same muscle? What is said of the rapidity of muscular contractions in other animals? 167. When are the involuntary muscles called into action? Why would it not have been safe to trust these important operations to the exclusive control of the will? 168. Give an instance where some of the muscles act under the government of the will, conjoined with those that are involuntary. 169. On what does the difference in muscular activity and strength depend?
170. Persons with small muscles, and largely developed nervous systems, will sometimes exhibit very great muscular power for a time; but it will not be of long continuance, unless the brain is functionally diseased, as in hysteria, delirium of fever, insanity, &c. Men of large muscles and small nerves can never perform feats of great strength; but they have the power of endurance, and are better capacitated for continued labor. Thus we cannot judge of the ability of persons to make exertions and continue them, by their stature alone. Strength, and the power of endurance, are the result of a combination of well-developed muscles, large nerves, and a full-sized, healthy, and active brain.
Observation. The muscles of fishes are large, and the nerves distributed to them, comparatively small. The muscles of birds are small, but their fibres are very compact. The nerves appropriated to the muscles that are called into action in flying, are large as well as numerous.
171. The contractile portion of a muscle is, in general, at a distance from the part to be moved. Thus the principal muscles that move the fingers are situated upon the forearm; and when the limb is nearly or quite extended, the angle formed by the part to be moved and the contractile muscles is small. Again, the attachment of the muscles to the part to be moved is near the joint that forms the fulcrum, (fig. 45.) By these arrangements there is a loss of power; but we are compensated for this disadvantage by increased celerity of movement, beauty of form, and adaptation of the limbs to the varied pursuits of man.