492 THE MECHANICAL FUNCTIONS. 



for the simple support of the trunk, is, on the 

 other hand, better adapted to give it those im- 

 pulses which are to effect its progressive move- 

 ments. The nature of those movements, and the 

 order in which they succeed each other, are dif- 

 ferent according to the peculiar mode of pro- 

 gression which the animal practises, the degree 

 of speed it is desirous of exerting, and the par- 

 ticular end it has in view. The paces of a qua- 

 druped usually distinguished, are the walk, the 

 trot, the gallop, the amble, and the bound. 



In slow walking, only one foot is raised from 

 the ground at the same moment, so that three 

 points of support always exist for sustaining the 

 weight of the body. If the centre of gravity be 

 situated, as it generally is, nearly over the middle 

 of the quadrangular base formed by the feet, 

 while they rest upon the ground, the first effect 

 to advance which the quadruped makes, propels 

 the centre of gravity forwards. This it accom- 

 plishes by pressing one of its hind legs against 

 the ground ; which leg being thus fixed by the 

 resistance it there meets with, becomes the ful- 

 crum of the first movements. The extensor 

 muscles of the limb are now exerted in giving 

 the body an impulse forwards. As soon as this 

 impulse has been given, the muscles which had 

 been in action are relaxed, and the leg is raised 

 from the ground, brought forwards, and laid down 

 close to the fore foot of the same side. This fore 



