CHAPTER III 

 WORK THE MUSCLES 



ALTHOUGH movement is not the only way in which the energy of 

 food is used up, it is the most striking and obvious way. It is 

 perfectly clear that work is done when we raise a weight or throw 

 a ball. 



On the other hand, it is unnecessary to remind the reader that 

 energy is expended in many other ways, as in the overcoming of 

 osmotic pressure, the formation of chemical compounds of a higher 

 potential than those from which they arise, and so on. 



In the present chapter we have to learn something about 

 muscular activity and its mechanism. 



' Suppose that we have a set of fibres attached at one end to a 

 bone and at the other end to another bone, which is capable of 

 moving by a hinge joint at the end of the former, and that the two 

 bones are placed so as to be in line with one another. It is plain 

 that if the fibres shorten, the two bones will be moved so as to form 

 an angle with one another, since in this position the line joining 

 a point on one to a point on the other is shorter (E., p 204). In 

 general, the action of a muscle when it enters into activity is 

 to bring closer together the points to which its two ends are 

 attached. One of these points is usually fixed, and is called the 

 "origin" of the muscle; the other is movable, and called the 

 " insertion." But, for special purposes, the parts may be reversed. 

 For example, the arm muscles may move the arm itself when the 

 body is fixed, or they may raise the body when the hands are 

 holding a fixed bar. A very great variety of movements is 

 rendered possible by the numerous muscles and bones connected 

 by joints, found in the vertebrate body. 



The first point to notice is that the designation " contraction " 

 is not really a correct one. The muscle does not change in volume, 

 but in shape. It becomes shorter and thicker. The increase in 

 thickness can easily be felt in the biceps muscle on the front of the 

 upper arm. 



We know, further, that if we attempt to move a very heavy 

 object our muscles enter into great activity, but are unable to* 



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