THE VERTEBRATE BODY 



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means of this simple reaction that all of the complicated movements of 

 an animal's body are performed. There are three kinds of muscles, 

 which are somewhat different in their reactions, so they will be studied 

 separately. 



The first of these is the striated or skeletal type of muscle. They are 

 called striated muscles because, when viewed under the microscope, they 

 are seen to be composed of many long- cylindrical fibers bearing small 

 cross striations. They are called skeletal muscles because they are 

 nearly all attached to some part of the skeleton. The movements which 

 take place at the movable joints are controlled by such muscles which 

 pass freely over the joints, but are attached to the bones on either side. 



Origin 



— Relaxed muscle 

 1 



Contracted muscle 



Insertion 



Fig. 22.7. Diagram illustrating the action of a flexor muscle. At the left the muscle 

 is shown relaxed ; at the right contracted. Note that the bone to which the muscle is 



inserted moves during contraction. 



The muscle may be attached directly to the bone or, more commonly, by 

 a connective tissue extension known as a tendon. When the muscle con- 

 tracts, the bones at either end may move, but one end will probably move 

 much more than the other. It is customary to call the point that moves 

 the most the insertion and the point that moves the least the origin of 

 the muscle. In the case of limb muscles, the origin is usually the end 

 nearest the trunk ; whereas the distal end of the muscle is the insertion. 

 For the muscle to be able to exert proper control over movements of 

 the joints it must not only be able to initiate movements by shortening 

 itself, but it must also be able to slow down movements by exerting a 

 resisting force as the muscle is stretched by other forces as it attempts 

 to contract. Without this property of muscle we would be unable to 

 walk downhill and all our movements would be jerky, as muscles con- 



