418 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



cases special processes projecting from the bones, the introduction of sesamoid 

 bones into the tendons, and the presence of pulley-like mechanisms. 



The contraction of a muscle causes the points to which it is attached to 

 approach one another, and the direction of the movement is often determined 

 by the direction in which the force of the contracting muscle is applied to the 

 bones. In the case of certain joints, however, the form of the joint-surfaces 

 and the method of attachment of the ligaments limits the direction of move- 

 ment to special lines; and when this is not the case the movement is usually 

 the resultant of the action of many muscles rather than the effect of the con- 

 traction of any one muscle. This question has been made the subject of 

 careful study by Fick. 1 



In the case of many muscles, both of the bones to which they are attached 

 are movable, and the result of contraction depends largely on which of the 

 extremities of the muscles becomes fixed by the contraction of other muscles. 

 Though most muscles have direct influence over only one joint, there are certain 

 muscles which include two joints between their points of attachment, and pro- 

 duce correspondingly complex effects. The accurate adjustment and smooth 

 graduation of most co-ordinated muscular movements is due to the fact that not 

 only the muscles directly engaged in the act, but the antagonists of these mus- 

 cles take pari in the movement. It would appear from the observations of 

 certain writers 2 that antagonistic muscles may be not only excited to contrac- 

 tion, but inhibited to relaxation, and that the tension of the muscles is thereby 

 accurately adjusted to the requirements of the movement to be performed. 

 The importance of the elastic tension and reflex tonic contractions of muscles 

 to ensure quick action, to protect from sudden strains, and to restore the parts 

 to the normal position of rest has been referred to elsewhere. 



The shape of tin; muscle has an important relation to the work which it 

 has to perform. A muscle consists of a vast number of fibres, each of which 

 can be regarded as a chain of contractile mechanisms. The longer the fibre, 

 the greater the number of these mechanisms in series and the greater the total 

 shortening effected by their combined action ; consequently, a muscle with 

 long fibres, such as the sartorius, is adapted to the production of extensive 

 movements. In order that a muscle shall be capable of making powerful 

 movements it is necessary that many fibres shall be placed side by side, as in 

 the case of the gluteus: " Many hands, light work." 



Standing. — In spite of the ease with which the many joints of the body 

 move, the ereel position is maintained with comparatively little muscular 

 exertion. It is an act of balancing in which the centre of gravity of the 

 body is kepi directly over the base of support. In the natural erect position 

 of the body the centre of gravity of the head is slightly in front of the oe- 

 cipito-atlantoid articulation, so that there is a tendency for the head to rock 

 forward, as is seen from the nodding of the head of one falling asleep. The 

 centre of gravity of the head and trunk together is such that the line of 

 gravity falls slightly behind a line drawn between the centres of the hip- 



1 Hermann's Handbuch 'In- Pkysiologie, 1871, Bd. i. pt. 2. S. I'll. 

 - Sherrington: Proceedings of the Royal Society, Feb., 1893, vol. liii. 



