160 SPECIAL PHYSIOLOGY. 



also, the muscular force is more conveniently reflected over the joints, 

 or other parts of the skeleton, than could be effected by the tender 

 sensitive muscle itself; in this case, the tendons often run in grooves 

 in the bones, lined by cartilage. It is, furthermore, obvious that by 

 the use of tendons, as extensions of the muscles, economy of muscular 

 tissue, and lightness and elegance of form around the joints, are se- 

 cured. In certain broad muscles, however (gluteus, great serratus), 

 the muscular fibres arise, at least in part, directly from the periostea! 

 covering of the bones. 



In undergoing contraction, muscles which are connected only with 

 soft parts, simply constrict or tighten those parts. Muscles which 

 are attached by one end to bone, and by the other to soft tissues, ex- 

 ercise a direct traction upon those parts, moving them in accordance 

 with their mechanical arrangements. The tendon of the superior 

 oblique muscle of the eyeball passes through a loop, which resembles 

 a pulley, the tendon being reflected in a new direction beyond it, so 

 that the movement impressed upon the eyeball is at an angle with the 

 line of direction of the contracting muscle. When muscles are at- 

 tached by both end.:, to bones, their action is after the manner of the 

 so-called levers of mechanics ; and either the efficient action of the 

 muscle may be exerted in the line of direction of its fibres and ten- 

 dons ; or, by the reflection of its tendon of insertion over some bony 

 point, its force may be exerted at a certain angle from its own direc- 

 tion. It is important also to notice that the tendons of insertion, and 

 sometimes also the tendons of origin, are attached to special eminences 

 of the bones, called processes; in which case, a muscle acts at an ad- 

 vantage, because its force ultimately operates, from or on to the bone, 

 in a line more or less perpendicular to the osseous surface, instead of 

 in a line nearly parallel with it, as would happen if the surface of the 

 bone were flat, instead of being so elevated. Muscles which pass over 

 the back of a joint, are usually called extensors, because they serve 

 to stretch, or extend, the part beyond the joint ; whilst those lying in 

 front of the joint are, for the opposite reason, called its flexors. 

 Other sets of muscles are known as pronators, supinators, rotators, or 

 levators and depressors, according to their respective uses. The 

 names of many muscles are derived from the number of their divisions, 

 their shape, position, points of origin and insertion, or from the direc- 

 tion of their fibres. In certain cases, a single muscle will contract to 

 perform a given action, as in the raising of the upper eyelid : some- 

 times two or three are called into action together, as in extending the 

 fore-arm at the elbow-joint (triceps, anconeus), or in flexing the same 

 (biceps, brachialis anticus, supiriator longus) ; but on the other hand, 

 much more commonly, many muscles concur in the production of a 

 single act ; and, indeed, indirectly, nearly every muscle of the body is 

 employed in most movements, either in fixing some parts of the skele- 

 ton, or in moving other parts. 



The rapidity of action of any given muscle, depends directly on the 

 length of its fibres, for the period of contraction is practically the same 

 in long or short muscles, the fibres of one of which may be twelve, 

 and of the other only three inches in length ; and, therefore, the amount 



