CHAPTER V 

 SKELETAL MUSCLE 



The skeletal muscles are of various sizes and shapes. 

 Many are elongated cylinders or prisms; others are in 

 the form of comparatively thin sheets. Most of the 

 muscles of the limbs can be fairly described as belong- 

 ing to the first class while the other type is found in the 

 walls of the abdomen. The longest muscles extend 

 perhaps as much as 18 inches between attachments, 

 the smallest recognized as individuals are tiny affairs in 

 the cavity of the middle ear. These are only a fraction 

 of an inch in length. 



When we observe a muscle in action we can usually 

 recognize that one . end is relatively fixed in position 

 while the other is moved by its contraction. The 

 comparatively stationary end is the origin, the movable 

 one is the insertion. The distinction is not always clear 

 and we can think of cases in which the two may seem 

 to be interchangeable, but there is generally obvious 

 reason to assume a certain type of action. A con- 

 venient choice for illustrative purposes is the biceps, on 

 the front of the upper arm. This is the muscle most 

 often exhibited with pride by the schoolboy. 



The biceps is attached to the shoulder-blade above and 

 to one of the forearm bones, the radius, below. It tapers 

 toward its extremities, and a little inspection shows 

 that these terminal parts are not contractile, but con- 

 nective tissue. They are tendons. The biceps has two 

 tendinous extensions at its upper end and takes its 

 name, " two-headed," from this fact. The intermediate 

 portion of the muscle is convex in contour and is called 

 its belly. This is the active, contractile part, but it is 

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