CHAPTEE II. 

 THE CONTRACTILE TISSUES. 



39. IN order that the blood may nourish the several tissues 

 it is carried to and from them by the vascular mechanism ; and 

 this carriage entails active movements. In order that the blood 

 may adequately nourish the tissues, it must be replenished by food 

 from the alimentary canal, and purified from waste by the excretory 

 organs; and both these processes entail movements. Hence before 

 we proceed further we must study some of the general characters 

 of the movements of the body. 



Most of the movements of the body are carried out by means 

 of the muscles of the trunk and limbs, which being connected with 

 the skeleton are frequently called skeletal muscles. A skeletal 

 muscle when subjected to certain influences suddenly shortens, 

 bringing its two ends nearer together; and it is the shortening, 

 acting upon various bony levers or by help of other mechanical 

 arrangements, which produces the movement. Such a temporary 

 shortening, called forth by certain influences and due as we shall 

 see to changes taking place in the muscular tissue forming the chief 

 part of the muscle, is technically called a contraction of the muscle ; 

 and the muscular tissue is spoken of as a contractile tissue. The 

 heart is chiefly composed of muscular tissue, differing in certain 

 minor features from the muscular tissue of the skeletal muscles, 

 and the beat of the heart is essentially a contraction of the 

 muscular tissue composing it, a shortening of the peculiar muscular 

 fibres of which the heart is chiefly made up. The movements of 

 the alimentary canal and of many other organs are similarly the 

 results of the contraction of the muscular tissue entering into the 

 composition of those organs, of the shortening of certain muscular 

 fibres built up into those organs. In fact almost all the move- 

 ments of the body are the results of the contraction of muscular 

 fibres, of various nature and variously disposed. 



