78 



General Biology 



expansion of circular and longitudinal muscles contained within the walls 

 of the blood vessels themselves. 



The outer surface of all muscles is covered by a connective tissue 

 membrane called fascia ( ), which is not very elastic. 



The fascia usually becomes thicker toward the end of the muscle, gradu- 

 ating in a dense, fibrous band called a tendon or, if this tendon is broad 

 and flat, an aponeurosis ( ). 



That part of the muscle most thoroughly attached — usually to a 

 relatively immovable part and most frequently toward the center of the 

 body — is called its origin. The more movable and distal attachment is 

 known as its insertion. 



The action of a muscle in con- 

 tracting is to draw origin and inser- 

 tion closer together. 



Whenever a muscle moves any 

 part of the body in its normal direc- 

 tion or as one may say, with the 

 joint, such movement is called 

 flexion ( ) ; against 



the joint, extension ( ). 



A muscle which pulls any limb or 

 portion of a limb away from the 

 central axis of the body is an ab- 

 ductor ( ), and 

 one which draws the limbs or their 

 appendages toward the center of the 

 body is an adductor ( ). 

 Rotators ( ) are 

 those which cause the limb to rotate 

 about its axis, such as those turning 

 the femur at the hip ; levators raise 

 a part, such as the lower jaw; and 

 depressors produce the opposite 

 movement. 



To know a muscle there are five 

 points which must be remembered: 



(1) Its Origin. 



(2) Its Insertion. 



(3) Its Relation to other struc- 

 s. 



(4) Its Innervation. 



(5) Its Action. 



Fig. 23. Different Types of Muscle-Fibres. 



A., embryonic striped muscle-fibre_ from the 

 tail of a tadpole, showing the nuclei nn., and 

 the protoplasm p., of the ccenocyte from which 

 the fibres are developed. The fibres exhibit 

 alternate dark and light bands, and in the 

 center of each dark band is a light line, the 

 line of Hensen. 



B., cardiac muscle-fibre showing the short 

 branched nucleated cells. 



C, a single cell from cardiac muscle-fibre 

 more highly magnified, showing the cross- 

 striation and the nucleus n. 



D., group of unstriped muscle-fibres from 

 the bladder: a., the nuclei; p., the granular 

 remains of the cell protoplasm; /., the longi- 

 tudinally striated contractile portion. (A and 

 D, from Bourne. B and C from Schafer.) 



tures. 



The following list will convey a clear and accurate idea of what 

 is essential in the study of the muscular system (Fig. 24). The rela- 



