THE MUSCULAR SYSTEM 



61 



the middle than at the ends, and the swollen middle 

 portion is called the belly. The belly is usually unat- 

 tached, but the two ends are secured to the 

 bones by tendons which are continuous 

 with the connective tissue of the muscle 

 (Fig. 42). 



Between the small fibers of the bundles 

 which make up a muscle is a little loose 

 areolar tissue in which are distributed the 

 blood vessels and nerves for the muscle. 



71. Muscle Cells or Muscle Fibers. It is in 

 the microscopic threads of the muscle that 

 the peculiar power of contraction lies. 

 These are variable in length and thick- 

 ness, but are said to average, in voluntary 

 muscles, - 5 ^ of an inch in diameter and 

 about one inch in length. They are cylin- 

 drical in shape, with rounded ends (Fig. 

 40), and as a rule do not branch. In 

 the muscles of the face and tongue, how- 

 ever, the muscle fibers divide into many Fi 42 _ Bice 

 branches. muscle. 



Each muscle fiber, or cell, consists of the 1 tendinous ends. 

 sarcolemma and a soft, semifluid material of 

 alternate light and dark disks, called the contractile sub- 

 stance. Just beneath the sarcolemma are several long 

 oval nuclei. 



72. Nerve Endings in Muscle Fiber. It is impossible to 

 treat of muscles and their action without including some 

 study of the other sort of irritable tissue, nervous tissue, 

 upon which muscular action depends. 



The sarcolemma of each muscle fiber is pierced by a 

 branch of a nerve fiber. The primitive sheath or neuri- 



