436 THE MECHANICAL FUNCTIONS. 



to effect its progressive movements. The nature of 

 those movements, and the order in which they suc- 

 ceed each other, are different according to the 

 peculiar mode of progression which the animal 

 practises, the degree of speed it is desirous of ex- 

 erting, and the particular end it has in view. The 

 paces of a quadruped 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 quadran- 

 gular base formed by the feet while they rest on 

 the ground, the first effort to advance which the 

 quadruped makes, propels the centre of gravity 

 forwards. This it accomplishes by pressing one of 

 its hind legs against the ground ; which leg, being 

 thus fixed by the resistance it there meets with, 

 becomes the fulcrum 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 hind foot is 

 raised from the ground, brought forwards, and laid 

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

 fore foot is next raised and advanced ; and then 

 the same succession of actions takes place with the 

 hind and the fore foot of the other side. 



An attentive examination of the conditions of 

 these successive positions will show that, amidst all 

 the changes which take place in the points of sup- 

 port, the stability of the body is constantly pre- 



