GENERAL STRUCTURE OF THE BODY. 23 



at once or be considerably delayed. It may last any- 

 where from a few moments to several days but gener- 

 ally lasts from twenty-four to thirty-six hours. It is 

 probably due to the formation in the muscle of myosin, 

 a substance which probably comes from myosinogen 

 in the living muscle and which is closely akin to the 

 fibrin of blood. Probably the myosin or what precedes 

 it causes clotting of the muscle just as fibrin or what 

 precedes it causes clotting of the blood. 



The muscles vary in shape in different parts of the 

 body, being long and slender in the limbs and broad and 

 flat in the trunk. They are attached chiefly to bones 



FIG. 6. Three voluntary muscle fibers from an injected muscle, showing 

 network of blood capillaries. (Hill.) 



but also to cartilages, ligaments, and skin, either by 

 means of tendons, which are cords or bands of white 

 inelastic fibrous tissue, or by means of aponeuroses, 

 membranous expansions of the same nature. Most 

 voluntary muscles consist of a belly and two ends or 

 tendons. The origin is the fixed point from which it 

 acts while the movable^ point upon which it acts is 

 known as its insertion. 



Action of the Muscles. When attached to bones, mus- 

 cles are distributed in three ways: 1. When it is neces- 

 sary to produce much motion rapidly, a short muscle is 

 used. 2. When a part needs to be moved far and 

 much contraction on the part of the muscle is, there- 

 fore, needed, the muscle is very long, as in the case of 

 the sartorius muscle, which shortens half its length. 3, 



