238 THE MUSCLES. 



For example, if a muscle is situated in an angle formed by two bones which 

 have a trochlean articulation, it will be a Jlexor ; but if it is placed behind the 

 summit of the angle, it becomes an extensor ; if it be located on the external side 

 of an enarthrosis, it is an abductor ; and when on the inner side of the joint, or 

 between it and the middle plane of the body, it is then an adductor. A muscle 

 may be rolled obliquely round the joint it controls — in passing, for instance, from 

 the inner face of one bone to the outer face of another forming an articulation — and 

 in such a case the flexors and extensors become also rotators if the diarthroses they 

 move permit rotation. 



Lastly, muscles applied more or less obliquely on bones united by a trochoid, 

 will pivot one of the bones on the other — for instance, the great oblique muscle 

 of the head. 



It is useless to multiply examples, for, the principle being once underetood, it 

 is easy in nearly every case to determine the uses of a muscle when its situation, 

 and the number and kinds of articulations included between its insertions, are 

 known. 



The result of the contraction of muscles 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 rectUmear mnscleB is the approxi- 

 mation of the bones to which they are attached. This approximation is usually 

 brought about by the displacement of a single bone — that which receives the 

 movable insertion of the muscle. Sometimes, however, the two bones move simul- 

 taneously, 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 circumscribes. 



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

 their point of inflection ; 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 bones are only levers moved by the 

 muscles. 



In the locomotory apparatus we find the three kinds of lever recognized by 

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

 an interfixed, or lever of the first class ; the foot, extended by the gastrocnemius 

 muscle, offers an example of the inter-resisting, or second kind, when the limb 

 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, 

 but at the expense of power. 



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

 at least at the commencement of their action ; a second circumstance which 

 diminishes their energy. 



Appendages of the Muscles. 



These are : 1. The enveloping or containing aponeuroses. 2. The serous or 

 mucous bursse. 3. The tendinous and synovial sheaths. 



