26 



ANIMAL LOCOMOTION. 



act, operate upon bones or something extraneous to them- 

 selves, and not upon each other. The muscles are folded 

 round the extremities and trunks of animals with a view to 

 operating in masses. For this purpose they are arranged in 

 cycles, there being what are equivalent to extensor and flexor 

 cycles, abductor and adductor cycles, and pronator and supina- 

 tor cycles. Within these muscular cycles the bones, or 

 extraneous substances to be moved, are placed, and when one 

 side of a cycle shortens, the other side elongates. Muscles 

 are therefore endowed with a centripetal and centrifugal 

 action. These cycles are placed at every degree of obliquity 

 and even at right angles to each other, but they are so dis- 

 posed in the bodies and limbs of animals that they always 

 operate consentaneously and in harmony. Vide fig. 5, p. 25. 



There are in animals very few simple movements, i.e. 

 movements occurring in one plane and produced by the action 

 of two muscles. Locomotion is for the most part produced 

 by the consentaneous action of a great number of muscles ; 

 these or their fibres pursuing a variety of directions. This is 

 particularly true of the movements of the extremities in walk- 

 ing, swimming, and flying. 



Muscles are divided into the voluntary, the involuntary, and 

 the mixed, according as the will of the animal can wholly, 

 partly, or in no way control their movements. The voluntary 

 .muscles are principally concerned in the locomotion of animals. 

 They are the power which moves the several orders of levers 

 into which the skeleton of an animal resolves itself. 



The movements of the voluntary and involuntary muscles 

 are essentially wave-like in character, i.e. they spread from 

 certain centres, according to a fixed order, and in given direc- 

 tions. In the extremities of animals the centripetal or con- 

 verging muscular wave on one side of the bone to be moved, 

 is accompanied by a corresponding centrifugal or diverging 

 wave on the other side ; the bone or bones by this arrangement 

 being perfectly under control and moved to a hair s-breadth. 

 The centripetal or converging, and the centrifugal or diverging 

 waves of force are, as already indicated, correlated. Similar 

 remarks may be made regarding the diff'erent parts of the body 

 1 Muscles virtually possess a pulling and pushing power ; the pushing 



