182 THE MUSCLES. 



to an immovable point. If the extremities of a muscle are attached to two 

 movable levers, its contraction will bring about the displacement of one or 

 other of these ; from this a movement is produced. 



The degree of shortening of a muscle varies, according to its being 

 entirely free, or having a resistance to overcome. The mean limit of this 

 shortening is about one-fourth the length of the muscular fibres ; from this it 

 will be understood that the movement produced by the contraction will be 

 in proportion to the length of the fibres ; though in this appreciation it will 

 be necessary to keep in mind the density and energy of the fibre, as well as 

 the intensity of the contractile stimulant. 



As each fibre represents a force independent in its action, it results that 

 we may judge of the power of a muscle by the number of its fibres, or its 

 volume. 



Muscles are often aided in their action by mechanical conditions : such as 

 the disposition of the levers on which they act, the direction of the muscular 

 fibres in connection with these levers, and, lastly, by the presence of lamellaB 

 or elastic cords. 



C. USES OF MUSCLES. There are flexor, extensor, abductor, adductor, 

 rotator, and other muscles, for all the movements of which the articulations 

 are the centre. 



To determine the functions or uses of the muscle, it is sufficient to know 

 their insertions, and the mode in which the bones furnishing these insertions 

 articulate with each other. 



The result of muscular contraction, being influenced by the form of their 

 principal axis, and the length and direction of their levers, it is necessary to 

 briefly examine these two points ; 



1. The immediate effect of the contraction of rectilinear muscles is the ap- 

 proximation of the bones to which they are attached. This approximation 

 is usually brought about by the displacement of a single ray : that which 

 receives the movable insertion of the muscle. Sometimes, however, the two 

 rays move simultaneously, or they are alternately fixed and movable. 



The first result produced by a curvilinear muscle is the straightening of 

 its component fibres ; after which it may act on the bony levers as do the 

 rectilinear muscles, if its contractile power be not entirely expended. When 

 a muscle is quite circular, its only action is to contract the opening it circum- 

 scribes. 



With regard to the inflected muscles, their action can only be estimated 

 from their point of inflexion ; they operate as if this point represented their 

 origin or fixed insertion. 



2. The muscular powers are submitted to the statical and dynamical laws 

 which govern the theory of levers , for the bony rays are only levers moved 

 by the muscles. 



In the locomotory apparatus we find the three kinds of lever recognised 

 by physicists. Thus the head, extended by the great complexus muscle, 

 represents an interflxed, or lever of the first class ; the foot, extended by the 

 gastrocnemii muscles, offers an example of the interresisting, or second kind, 

 when this member remains fixed on the ground ; lastly, the lower jaw raised 

 towards the upper by the masseter muscle, forms an interpuissant or third 

 kind. 



It is worthy of remark that the arm of resistance in the bony levers is 

 always extremely long ; a circumstance which favours speed and the extent of 

 movement at the expense of power. 



On the other hand, muscles are rarely perpendicular to the arm of their 



