CHAPTER II. 



THE CONTKACTILE 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 

 which, by acting upon various bony levers or by help of other 

 mechanical arrangements, produces the movement. Such a tem- 

 porary 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 musclar 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 movements of the body are the results of the contraction of 

 muscular fibres, of various nature and variously disposed. 



