The bones of the skeleton form the framework of the body. Joints are obviously arranged to admit of motion, they do not originate it. That function is relegated to the muscles, which form the masses of flesh covering the bones. Muscular tissue possesses the peculiar property of contraction, and the effect of contraction is to pull the parts to which muscles are attached nearer to each other.
As nearly all the joints of the horse are simply hinge-joints capable only of flexion and extension, it would be expected that only flexor and extensor muscles would be required. It is a fact, however, which Sir W. Flower comments on in his work on the horse, that many more muscles exist in the limbs of the animal than would be thought necessary for the very simple functions which they have to perform. It would appear that the reduction of bones to a rudimentary condition, as in the case of the ulna and the fibula, or their entire loss, as in the case of four of the toes, has taken place more thoroughly than, and in advance of, that of the muscles which were originally connected with these bones, many of which linger, as it were, behind, though with new relations and uses, sometimes in a most reduced, and almost, if not quite, function-less condition, and sometimes even with completely changed structure.
Dr. G. E. Dobson remarks in this connection that if no other evidence were obtainable of the descent of the horse from five-toed ancestors, the condition of the muscles of the foot would be a sufficient indication.
Most of the muscles of the forearm of the five-toed mammal are still represented in the extremities of the horse; the proper extensor even of the fifth digit survives, although both its position and special function have been completely altered.
" In the human hand there are fifteen muscles which have special functions in the complicated movements of the organ. Only five1 of them remain in the horse, four in a very reduced condition, two interossei, and two lumbricales. The fifth muscle, a short flexor muscle, called in man the first palmar interosseous, is referred to as a remarkable instance of a structure not becoming rudimentary and useless, but being completely diverted from its original purpose, its function and its structure also being changed. In the horse the modified muscle is entirely transformed, and in its new form is known as the suspensory ligament - a strong fibrous band lying at the back of the cannon-bone, being attached to its upper extremity, and dividing at the lower end into two portions which spread over the fetlock-joint and are inserted partly into the sesamoid bones and partly into the extensor tendon on the first phalanx."
1 Others have been discovered in later dissections.
The most interesting point, however, remarks M. D. J. Cunningham, in connection with its structure is that it bears its history on its face. Almost invariably two thin streaks of striated muscular fibre are to be found on its superficial surface, leading down to the two inferior divisions. On examining its deep surface two very distinct strands of pink, fleshy tissue are always observed extending throughout the entire length of the ligament. These consist in each case of short oblique striated fibres, and are presumed to represent the two heads of the muscle called the flexor brevis, not yet converted into fibrous tissue. It is hardly necessary to suggest that muscular fibre in such a form and position, and with such surroundings, cannot possess any functional value, that is, does not serve any really useful end. Indeed, it can only be looked upon as a vestigial tissue which is slowly passing away.