432 - Multicellular Animals, Especially Man 



BALL AND SOCKET JOINT (SHOULDER). 



HUMERUS. 



ORIGINS . 



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 



-BICEPS 

 -TRICEPS 



-PECTORAUS MAJOR 



-EXTERNAL OBLIQUE 



-SHEATH OF RECTUS 

 ABDOMINIS 



' jjrf — SARTORIUS 

 iU-GRACILIS 



-QUADRICEPS FEMORIS 

 GASTROCNEMIUS 



Fi9- 



24-2. Superficial muscles of man, anterior as- 



muscles is very great. The extensor muscles pect. (Courtesy A. J. Nostrom Co.) 



