. Principles of modern biology. Biology. 432 - Multicellular Animals, Especially Man BALL AND SOCKET JOINT (SHOULDER). HUMERUS. TRICEPS HINGE JOINT (ELBOW) INSERTION Fig. 24-1. Two muscles of the human arm, hav- ing a more or less antagonistic action. Note that the biceps tends to bend (flex) the elbow, while the triceps straightens (extends) the joints. The biceps, however, has another main action, namely, to rotate the forearm. bend, or straighten, the joint. Ii movement is to occur, the extensors must relax during the time that the flexors are contracting; and conversely, the flex
. Principles of modern biology. Biology. 432 - Multicellular Animals, Especially Man BALL AND SOCKET JOINT (SHOULDER). HUMERUS. TRICEPS HINGE JOINT (ELBOW) INSERTION Fig. 24-1. Two muscles of the human arm, hav- ing a more or less antagonistic action. Note that the biceps tends to bend (flex) the elbow, while the triceps straightens (extends) the joints. The biceps, however, has another main action, namely, to rotate the forearm. bend, or straighten, the joint. Ii movement is to occur, the extensors must relax during the time that the flexors are contracting; and conversely, the flexors must relax in syn- chrony with the contracting extensors—as the joint undergoes bending and unbending. Each movement of the body depends upon a reciprocal action of antagonistic muscles, but the maintenance of posture demands that the antagonistic muscles contract synchro- nously. While a person stands, for example, both the flexors and extensors of the knee are contracting simultaneously, locking the joint and converting the leg into a rigid pil- lar to support the weight of the body. More- over an important function of a set of mus- cles is to arrest the movements of the an- tagonistic muscles. Thus, obviously, the con- tractions and relaxations of the interacting muscles must be coordinated by the nervous system, and each muscle remains inactive until it receives excitations from the nervous system. In an average man, the weight of the mus- cles constitutes more than 50 percent of the body weight; and the strength of the larger. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Marsland, Douglas, 1899-. New York, Holt, Rinehart and Winston
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