A treatise on physiology and hygiene for educational institutions and general readers .. . scle, or group ofmuscles, which brings the limb back to itsplace. When they act alternately, a to-and-fromovement results. When a joint is bent, themotion is called flexion ; and when it is madestraight again, it is called extension. Whenboth sets act equally, and at the same moment,no motion is produced, but the body or limb ismaintained in a fixed position: this occurswhen we stand erect. The muscles which pro-duce extension are more powerful than thoseopposite to them. 4. The muscles are also distingu
A treatise on physiology and hygiene for educational institutions and general readers .. . scle, or group ofmuscles, which brings the limb back to itsplace. When they act alternately, a to-and-fromovement results. When a joint is bent, themotion is called flexion ; and when it is madestraight again, it is called extension. Whenboth sets act equally, and at the same moment,no motion is produced, but the body or limb ismaintained in a fixed position: this occurswhen we stand erect. The muscles which pro-duce extension are more powerful than thoseopposite to them. 4. The muscles are also distinguished as thevoluntary and involuntary muscles, accordingas they are, or are not, under the control of thewill. The heart is an example of the involun-tary variety. We cannot change its action inthe least by an effort of the will. When wesleep, and the will ceases to act, the heart con-tinues to beat without cessation. The volun-tary muscles, on the other hand, are such as areused only when we wish or will to use them— as the muscles of the hand or arm (Figs. 11 and 12). {ReadNote 1.). Fig. 11.—A, Biceps muscle of the arm ; B, C,Its tendons. 1. The Perfection of the Human Hand.—Gordy counts thirty-fourdistinct movements of the hand, and if we include the combinations of thesedifferent movements, we shall reach a much higher number. Properly speak-ing, the hand belongs to man alone, and its form does not permit us to con-sider it an organ of locomotion, as is the case with certain animals most closelyresembling man. Nothing gives a more complete idea of the perfection of the 3. Arrangement of the muscles ? Their action ? Flexion and extension ? Action of themuscles when we stand erect ? 4. Kinds of muscles? The voluntary? Involuntary? The heart? Give the hand ? Arm ? THE MUSCLES. 33 5. The Tendons.—Tendons, or sinews, are the extremities ofmuscles, and are firmly fastened upon the bone. They are very
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