78 



GENERAL BIOLOGY 



pansion 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, 



and usually becomes thicker toward the end of the muscle, graduating 

 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 ( ) aro 

 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. 



Its Relation to other struc- 



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 ex- 

 hibit alternate dark and light bands, and in 

 the centre 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.) 



(3) 

 tures. 



(4) 



(5) 



Its Innervation. 

 Its Action. 



The following list will give the student a clear and accurate idea 

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

 relation of each muscle to surrounding structures can be obtained only 



