Muscles and Motion. 21 



and partitions would remain, and show the form of the 

 muscle just as the partitions remain in a squeezed orange 

 or lemon. The connective tissue forms a framework for 

 all the soft tissues of the body, and if their working cells 

 were removed, the connective tissue would remain, and 

 show, more or less completely, the form of the part. Con- 

 nective tissue may be called, therefore, the skeleton of the 

 soft tissues. Muscle consists, then, of soft transparent 

 tubes, filled with a semi-fluid muscle substance. By scrap- 

 ing the surface of a steak the muscle substance may be 

 obtained, leaving the connective tissue. This is a good 

 way to get the nutritious part of beef for an invalid. 



Importance of Muscles. The muscles make up nearly 

 half the weight of the body. This fact of itself should 

 lead us to conclude that the muscles are of great impor- 

 tance. Muscles are used in nearly every action of the 

 body, not only in locomotion, but in respiration, circulation, 

 digestion, speech, etc. It is very necessary that at the 

 beginning we understand their action. 



Duration of Muscle Shortening. A muscle cannot be 

 kept shortened for any great length of time. If one holds 

 the arm out horizontally as long as he can, he soon feels 

 fatigue, later pain, and he is likely to feel a soreness in the 

 muscle for several days afterward. The law of muscle 

 action is to alternate periods of rest with periods of action. 

 In many exercises, as in walking, the limbs act alternately, 

 one resting or recovering while the other works. 



Alternate Action of Flexors and Extensors. When the 

 biceps muscle shortens and bends the arm, the triceps 

 lengthens (see Fig. 16). When the triceps shortens, as in 

 straightening the arm, the biceps lengthens. If the biceps 

 and triceps both shorten at the same time, and with equal 



