120 



ANATOMY AND PHYSIOLOGY 



the insertion of the muscle fibers, and by this means only a narrow surface 

 is required for insertion upon the bone. But for arrangements like these, 

 the skeleton would of necessity be inconveniently large. 



The blood-vessels bring the nutritive fluid which, in the tissue- 

 spaces, bathes each little fiber, and is gathered up by the lymph- 

 vessels. One-fourth of the blood in the body is in the muscles. 



The nerves bring to each fiber its natural stimulus to action. 



The work of muscle tissue is done in the fiber cell. This, 

 when stimulated, contracts, bringing the two ends of the fiber 

 nearer to each other, and naturally the fiber swells as it shortens. 

 So with the myriad of fibers in a muscle; when they contract, the 

 muscle swells and shortens (Fig. 98) illustrates the changes pro- 



FIG. 98. SHOWING CHANGE OF SHAPE IN CONTRACTION. (Brubaker.) 



duced). This results in motion, which appears as the organs 

 attached are moved. One-third of the body weight is muscle 

 tissue. 



AlFskeletal muscles are so attached as to be tense, that is, they 

 are just a little stretched, so that it is easier for them to act than 

 not. (A cut across a muscle releases it from tension and leaves a 

 gaping wound.) See p. 123, tension and tonus. 



The actions of muscles are regulated by their attachments, and 

 the function is often expressed in the name. If muscles or their 

 tendons pass in front of a joint, for instance, causing flexion, they 

 are frequently called flexors; or if they pass behind such joints, 

 they may be called extensors; and so with other muscles and joints. 

 Examples: Flexors of the wrist, extensors of the fingers, etc. 

 Many other examples will occur to the student, as abductors, 

 adductors, pronators, etc. 



