TYPICAL STRUCTURE OF ORGANS 



33 



two portions: a central, thick, red part, known as the belly, 

 soft when the muscle is at rest, hard when it is contracted ; 

 and cordlike strings, or tendons, two at the upper end and one 

 at the lower, by means of which the muscle is attached to two 

 bones of the shoulder girdle and to one of the forearm. When 

 the belly of the muscle shortens, the points a and b are brought 

 closer together and the arm is bent, or flexed, at the elbow. 

 This drawing together, or contraction, is the special work, or 

 function, of muscles in general. 



Everyone has seen the cross 

 section of a muscle in a raw 

 beefsteak. This shows that the 

 muscle as a whole is surrounded 

 by a sheath of connective tissue 

 which contains more or less fat ; 

 septa pass inwards, dividing the 

 muscle into lesser red masses 

 known as fasciculi, or bundles, 

 and these are further subdivided 

 into secondary fasciculi by sec- 

 ondary septa, very much as the 

 gland is subdivided into lobules. 



A longitudinal section shows 

 that the fasciculi run from ten- 

 don to tendon, and microscopic 

 examination proves that the general connective tissues of the 

 belly of the muscle are continuous with that of the tendon. 

 The tendon itself is a peculiarly strong and inextensible 

 variety of connective tissue consisting chiefly of parallel 

 fibers which are specially fitted to transmit to the bone the 

 pull of the belly of the muscle. 



6. The muscle fibers. Examination of the structure of 

 one of the finer fasciculi in the belly of the muscle shows 

 that it is composed of threads, or fibers, which at first sight 

 differ greatly from the secreting cells of the gland. These 



FIG. 23. The biceps muscle of 

 the arm 



The resting condition is shown by 

 the solid lines, the contracting con- 

 dition by the dotted lines 



