. THE MUSCULAR SYSTEM 65 



as well as those of all sizes and lengths between. Many 

 muscles which we might at first think to be long are 

 really short, but appear long because of the long tendons 

 by which they are attached to the bones. Many of the 

 muscles which move the fingers, for example, have their 

 bellies in the forearm, and are attached to the small bones 

 of the fingers by long, slender tendons. The force with 

 which muscles contract is sometimes very great. A human 

 muscle one square centimeter (.15 square inch) in section 

 can raise a weight of 5670 grams or 200 ounces. 



When a muscle contracts, its two ends are drawn nearer 

 together, and hence draw toward each other the parts to 

 which the ends of the muscle or its tendons are attached ; 

 the belly of the muscle becomes swollen, and in strong 

 contraction the whole muscle becomes tense and hard. 



78. Contraction is speedily followed* by relaxation. The 

 stimulating force ceases to be supplied, and the muscle 

 returns to a state of rest. If the hand has been raised by 

 the contraction of the biceps muscle on the inner side of 

 the humerus, it will fall under the action of gravitation 

 when that contraction ceases and the muscle becomes 

 passive. In order that the hand may be drawn down 

 with force the action of the triceps muscle at the back of 

 the humerus is needed. Muscles can give a powerful pull, 

 but they cannot push. Very generally, therefore, they 

 are arranged so that muscles which cause movement in 

 one direction are opposed by those which cause movement 

 in the opposite direction. 



79. Dead Muscle. - - The muscles of a dead body, or 

 muscles which have been removed from a living body, 

 gradually undergo a marked change, which results in 

 the stiffening known as rigor mortis. That which was 

 translucent becomes more opaque, most of the natural 



MACY'S PHYS. 5 



