The first phalanx (os suffraginis) normally sets at an angle of about 50 to 55 degrees from a horizontal plane while weight is being supported.  Its distal end articulates with the second or median phalanx (os corona) and forms the proximal interphalangeal (pastern or suffraginocoronary) joint.  This also, is a ginglymus joint, having but slight lateral motion, and that only when it is in a state of flexion.  A rather broad articular surface—­from side to side—­exists here, lessening the strain on the collateral ligaments somewhat.  Dorsal flexion is checked by the flexor tendons and dorsal ligaments.  Volar flexion is restrained by the extensor tendons.

The distal end of the second phalanx (os corona) has but slight lateral motion and this is manifested principally when it is in a state of volar flexion.  Undue dorsal flexion is prevented by the deep flexor tendon (perforans) and volar flexion is inhibited by the extensor of the digit (extensor pedis).  Thus it is seen, that when the leg is a weight-bearing member, weight is supported by the bony framework whose constituent parts are joined together by ligaments and tendons and each one of the several bones articulates in such manner that the joint is locked.  The articular parts of bones rest upon or against an inhibitory apparatus, and are slightly flexed, as in the carpus, or considerably flexed such as in the fetlock joint when weight is being supported.  In the first instance, for example, the flexors of the carpus and the superior check ligament assisted by the flexors of the phalanges constitute the inhibitory apparatus.

It will be noted that provision for weight bearing is so arranged that muscular energy is not required except in the matter of suspension of the body between the scapulae and here tonic impulses only are necessary to maintain an equilibrium[6], yet in every instance where weight is not supported by bones, inelastic ligaments or tendinous structures relieve the musculature of this constant strain.  This explains the fact that some horses do not lie in the stall, yet in spite of their constant standing position, they are able to rest and sleep.

The student of lameness is interested in the function of the legs in the role of supporting weight and as propelling parts, and not particularly in the capacity of these members for inflicting offense or as weapons of defense.  Yet, in the exercise of their functions other than that of locomotive appliances, injury often results, but usually it is the recipient of a blow that suffers the injury, such as an animal may receive upon being kicked.  Therefore, we do not often concern ourselves with strains or other injuries that the subject experiences as the result of efforts put forth in kicking or striking.  Where such injuries occur, however, a diagnosis is established by making use of the principles heretofore discussed.

As propelling members the front legs bear weight and are advanced alternately when the horse is walking or trotting—­in cantering this is not so.  When the normal subject travels in a straight line, at a walk or a trot, the length of the stride is the same with the right and left members.  The stride of the right foot then, for example, is equally divided by the imprint of the left foot, in the normal horse, when traveling at a walk and in a straight line.