13 ON THE WALK OF QUADRUPEDS. 



complexities of any system, to perform her work by means sure and 

 certain, and at the same time the easiest and most simple. 



The step of animals does not include the motion of the whole body 

 moved forward, with equal rapidity and in the same direction, as in 

 flying, leaping, or creeping, but it is rather the motion arising from the 

 transference of some parts of the body, which are supported by others 

 at rest, and in this manner an animal is moved forward in walking, 

 standing and moving by turns. For this reason, the standing and 

 moving which includes the step of animals, cannot be tottering and 

 unsteady, but firm and secure, and ought to be performed with the 

 least yet sufficient labour of the muscles. But as the attitude of a 

 quadruped in walking must be free from the risk of falling, it is neces- 

 sary that the body of the animal be supported upon more than two 

 columns I mean upon three or four, within which the line perpendi- 

 cular to the centre of gravity may fall. It is very evident that the 

 step of quadrupeds is performed in the manner explained in this pro- 

 position. 



It is moreover necessary that the support of the animal should be 

 accompanied with the least labour and pain to itself, and this is 

 accomplished by the legs serving as columns, which, on account of 

 their hardness and obtuseness of feeling, easily support the superin- 

 cumbent weight, without any remarkable uneasiness. 



Besides, the motion of the animal body is easily performed, because 

 its whole mass is not raised from the ground at once, one foot only 

 being lifted and moved forward. This is performed by first pressing 

 upon the ground with one of the hind feet, and not by one of the fore 

 feet, for if the former was lengthened while the latter rested upon the 

 ground, the centre of gravity would fall behind ; but, on the contrary, 

 by one of the hind feet being lengthened, the fore foot is moved for- 

 wards like a gladiator's spear (ad instar contC); whence it happens that 

 the whole mass of the animal is moved forward by the bending of three 

 erect columns, not different from the manner in which running upon 

 wheels is performed. Next by raising from the ground the same hind 

 foot, the joints being bent by the muscles, afterwards by the motion of 

 the fore foot of the same side as has been mentioned, these advantages, 

 I say, show the necessity of such an operation. 



