I^nITKODUCTION. 



19 



which, if supported, the body will be supported, however it 

 may be situated in other respects ; and hence the effects pro- 

 duced by or upon any body are the same as if its whole mass 

 were collected into its centre of gravity. 



The attitudes and motions of every animal are regulated 

 by the positions of their centres of gravity, which, in a state 

 of rest, and not acted upon by extraneous forces, must lie in 

 vertical lines which pass through their basis of support. 



In most animals moving on solids, the centre is supported 

 by variously adapted organs ; during the flight of birds and 

 insects it is suspended ; but in fishes, which move in a fluid 

 whose density is nearly equal to their specific gravity, the 

 centre is acted upon equally in all directions." ^ 



As the locomotion of the higher animals, to which my 

 remarks more particularly apply, is in all cases effected by 

 levers which differ in no respect from those employed in the 

 arts, it may be useful to allude to them in a passing way. 

 This done, I will consider the bones and joints of the skeleton 

 which form the levers, and the muscles which move them. 



" The Lever. — Levers are commonly divided into three kinds, 

 according to the relative positions of the prop or fulcrum, the 

 power, and the resistance or weight. The straight lever of 

 each order is equally balanced when the power multiplied by 

 its distance from the fulcrum equals the weight, multiplied by 

 its distance, or P the power, and W the weight, are in equi- 

 librium when they are to each other in the inverse ratio of 

 the arms of the lever, to which they are attached. The 

 pressure on the fulcrum however varies. 



In straight levers of the first kind, the fulcrum is between 

 the power and the resistance, as in fig. 1, where F is 

 the fulcrum of the lever AB ; P is the power, and W the 

 weight or resistance. We have P : W : : BF : AF, hence 

 1 Cyc. of Anat. and Phy., Art. '^Motion/' by John Bisliop, Esq. 



Fig. 1. 



