The paces of the horse cannot well be understood without reference to the means by which locomotion is brought about.

In the anatomical portion of this work frequent reference will be found to the origin and insertion of muscles, and the attachments of tendons and ligaments. Attention is called to the fixed points upon which muscles act by the shortening of their body or "belly". The stimulus of the will or other agents put in action a force which causes a contraction of muscular fibre, resulting in approximation of the fixed points of origin and insertion.

A familiar example of this action is seen when the human biceps, by its bellying or contraction, changes the contour of the limb and brings the hand to the point of the shoulder.

The horse may be viewed as a living mechanism, a series of pulleys and levers attached to bones, and having the ground for a fulcrum.

The comparison is not, however, so complete as some Continental writers have striven to prove. Marey says: " The comparison between ordinary machines and animated motive powers will not have been made in vain if it has shown that strict relations exist between the form of the organs and the character of their functions; that this correspondence is regulated by the ordinary laws of mechanics; so that when we see the muscular and bony structure of an animal we may deduce from their form all the characters and functions they possess".