584 FUNCTION OF THE MUSCLES IN THE BODY. 



by pressure from without or by partial contraction of a number of circu- 

 lar fibers, muscle-fibers that pass through the valley of the excavation 

 to the surrounding borders may obliterate the depression by partial 

 contraction, thus partially dilating the enclosed cavity, and converting 

 the concave aspect of the depression into a smaller, plane one. The 

 various layers are innervated from the same motor source, a fact that 

 likewise supports the view of their homologous action. 



2. The sphincters encircle an opening or a short canal, which is either 

 narrowed or firmly closed by their action; for example the iris, orbicu- 

 laris palpebrarum, orbicularis oris, sphincter pylori, sphincter ani, 

 sphincter vulvae, sphincter urethrae. 



B. Muscles with Definite Origin and Insertion. 



1. The origin is completely fixed when the muscle is in action. The 

 course of the muscle-fibers to their insertion is such that during con- 

 traction the insertion approaches the origin in a straight line; for ex- 

 ample the attollens, attrahens, and retrahens auriculse, and the rhom- 

 boids. In the case of some of these muscles, the insertion is lost in soft 

 structures, which then follow the line of traction; for example the azygos 

 uvulae, the elevator of the soft palate, most of the facial muscles arising 

 from the bones and inserting into the skin, the styloglossus, stylophar- 

 yngeus, and others. 



2. Both Origin and Insertion are Movable. Under such circum- 

 stances the movements of both points are inversely as the resistances 

 that have to be overcome by the movement. In this connection it 

 should be borne in mind that these resistances can often be voluntarily 

 increased either at the origin or at the insertion. Thus, for example, the 

 sterno-cleido-mastoid may act either as a depressor of the head, or, if the 

 head be fixed, as an elevator of the chest ; the pectoralis minor may act 

 either as an adductor and depressor of the shoulder or, if the shoulder 

 be fixed, as an elevator of the third, fourth, and fifth ribs. 



3. Some muscles with a fixed origin undergo a deviation from the 

 straight line in the further course of their fibers or their tendons. This 

 may be merely a slight curving, as in the occipito-frontal or the elevator 

 of the upper eyelid; or it may be an angular deflection of the tendon 

 around a firm prominence, so that the muscular traction is made in an 

 entirely different direction, namely as if the muscle acted from this pro- 

 cess directly on its insertion. Examples of the latter are the superior 

 oblique muscle of the eyeball, the tensor tympani, tensor veli palatini, 

 obturator internus. 



4. Many muscles of the extremities act upon the long bones as upon 

 levers: (a) The muscle may act upon a lever with a single arm, the 

 insertion of the muscle and the weight being situated upon the same side 

 of the point of support, or fulcrum, for example the biceps, the deltoid. 

 The point of application of the force, under such circumstances, is often 

 situated close to the fulcrum. By this means, the rapidity of the move- 

 ment during contraction of the muscle is greatly increased at the ex- 

 tremity of the arm of the lever; for example, in throwing, the hand may 

 move at a rate exceeding 22 meters a second; but force is lost. This 

 arrangement, however, has the advantage that with lesser contraction 

 of the muscle its force is diminished less than it would be if the contrac- 

 tion were more marked, (b) The muscles may act upon the bones as 

 upon levers with two arms, the point of application of the force (muscu- 

 lar insertion) being situated upon the other side of the fulcrum than the 



