The heart fibers are exceptional ; they are the only invol- 

 untary muscle fibers that are striped (Fig. 41). 



THOUGHT QUESTIONS. Classification of Some of the Muscles. 

 Copy the following list and mark / for involuntary and V for voluntary 

 after the appropriate muscles. 



Muscles for chewing. Muscles of gullet. Muscles of the heart. 

 Muscles that move arms. Muscles for breathing. Muscles in the skin 

 that cause the hair to stand on end. Muscles that move eyelids. 

 Muscles that contract pupil of eye. Muscles for talking. Muscles 

 that contract and expand the arteries (in blushing and turning pale). 

 Muscles that move eyeball. Muscles that give expression to the face. 



Tendons. The connective tissue which binds the fibers of 

 muscles into bundles, and forms sheaths for the bundles, 

 extends beyond the ends of the muscles and unites to form 

 tough, inelastic white cords called tendons. Some muscles 

 are without tendons, and are attached directly to bones. 

 Study the figures and find examples of this (see Figs. 

 44, 75). To realize the toughness of tendons, feel the 

 tendons under the bent knee or elbow, where they feel 

 almost as hard as wires. The tendons save space in places 

 where there is not room enough 

 for the muscles, and permit the 

 bulky muscles to be located where 

 they are out of the way. Wher- 

 ever the tendons would rise out of 

 position when a joint is bent, as 

 at the wrist and ankle, they are 

 bound down by a ligament. 



Arrangement of Voluntary Mus- 

 cles. Circular muscles, called 

 sphincter muscles, are found around 

 the mouth and eyes. Muscles that 

 extend straight along the limb either bend it and are called 

 flexors, or straighten it and are called extensors. Most of 



FIG. 43. (For blackboard.) 

 BICEPS relaxed and contracted. 



