INTRODUCTION. 



27 



of the serpent when creeping, of the body of the fish when 

 swimming, of the wing of the bird when flying, and of our own 

 extremities when walking. In all those cases the moving 

 parts are thrown into curves or waves definitely correlated. 



It may be broadly stated, that in every case locomotion is 

 the result of the opening and closing of opposite sides of 

 muscular cycles. By the closing or shortening, say of the 

 flexor halves of the C3^cles, and the opening or elongation of 

 the extensor halves, the angles formed by the osseous levers 

 are diminished ; by the closing or shortening of the extensor 

 halves of the cycles, and the opening or elongation of the 

 flexor halves, the angles formed by the osseous levers are 

 increased. This alternate diminution and increase of the 

 angles formed by the osseous levers produce the movements 

 of walking, swimming, and flying. The muscular cycles of 

 the trunk and extremities are so disposed with regard to the 

 bones or osseous levers, that they in every case produce a 

 maximum result with a minimum of power. The origins 

 and insertions of the muscles, the direction of the muscles and 

 the distribution of the muscular fibres insure, that if power 

 is lost in moving a lever, speed is gained, there being an 

 apparent but never a real loss. The variety and extent of 

 movement is secured by the obliquity of the muscular fibres 

 to their tendons ; by the obliquity of the tendons to the bones 

 they are to move ; and by the proximity of the attachment 

 of the muscles to the several joints. As muscles are capable 

 of shortening and elongating nearly a fourth of their length, 

 they readily produce the precise kind and degree of motion 

 required in any particular case.^ 



The force of muscles, according to the experiments of 

 Schwann, increases with their length, and vice versa. It is a 

 curious circumstance, and worthy the attention of those in- 

 terested in homologies, that the voluntary muscles of the 



power being feeble and obscured by the flaccidity of the muscular mass. In 

 order to push effectually, the pushing substance must be more or less rigid. 



^ The extensor muscles preponderate in mass and weight over the flexors, 

 but this is readily accounted for by the fact, that the extensors, when limbs 

 are to be straightened, always work at a mechanical disadvantage. This is 

 owing to the shape of the bones, the conformation of the joints, and the 

 position occupied by the extensors. 



