340 THE MECHANICAL FUNCTIONS. 



the trunk, is, on the other hand, better adapted to give it 

 those impulses which are to effect its progressive movements. 

 The nature of those movements, and the order in Which they 

 succeed each other, are different according to the peculiar 

 mode of progression which the animal practises, the degree 

 of speed it is desirous of exerting, and the particular end 

 it has in view. The paces of a quadruped usually distin- 

 guished, 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 mid- 

 dle of the quadrangular base formed by the feet, while they 

 rest upon 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 move- 

 ments. 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 ac- 

 tion 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 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 suc- 

 cessive positions will show that, amidst all the changes which 

 take place in the points of support, the stabilit}^ of the body 

 is constantly preserved. It is an elementary proposition in 

 mechanics that all that is necessary for ensuring the sup- 

 port of a body on any given base, is that the vertical line 

 drawn from the centre of gravity shall fall within that base. 

 When the animal is standing, the feet form a quadrilateral 

 base, and the centre of gravity is in a vertical line passing 

 either through the centre of the base, or, as, for the reasons 



