24 GENERAL STRUCTURE OF THE BODY. 



Finally, where less distance has to be covered but 

 greater power is required, tendons are used, as in this 

 case the contraction is powerful but does not carry 

 the part far. 



In performing the mechanical work of the body the 

 muscles are aided by the fact that the bones, to which 

 they are largely attached, are set together loosely and 

 form a set of levers, on which the muscles act to perform 

 certain definite acts. All three classes of levers occur: 

 1. where the fulcrum is between the weight and the 

 power, as in the case of the head, which is balanced by 

 the muscles of the neck on the vertebrae; 2. where 

 the weight is between the fulcrum and the power, as 

 when a person raises himself upon his toes; and 3. 

 where the power is between the fulcrum and the weight, 

 as when the biceps is used to raise a weight held in the 

 hand. The erect position of the body is difficult to 

 maintain because the center of gravity is high up, and it is 

 by the contraction of many muscles in the legs, thighs, 

 back, abdomen, and neck that the body is balanced 

 upright upon the feet. 



Physiology of Muscle. Irritability or sensitiveness to 

 stimulation and contractility or the power to contract 

 are the two most important functions of muscle. Con- 

 traction occurs in response to nervous energy brought 

 by the nerves, a nerve filament going to each muscle 

 fiber, into which it plunges, its substance being lost and 

 its sheath becoming continuous with that of the mus- 

 cle fiber. Any irritant, as heat, electricity, etc., when 

 applied to the nerve, causes the muscle to contract. 

 Moreover, muscle has an irritability of its own and 

 can contract independently of the nervous system. In 

 contracting it shortens and thickens, bringing the two 

 ends closer together, and becomes firm and rigid. The 

 amount of contraction depends upon the strength of the 

 stimulus and the irritability of the muscle. The mini- 

 mal stimulus is the least stimulus that will cause a con- 

 traction and the maximal is one that will cause the great- 



