SPECIAL FEATUEES IN STRUCTUEE 491 



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 ijerform. 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 hind -limbs of the horse the two flexors of the great toe and 

 the next one are both present with well-developed tendons united in the 

 foot, as in the greater number of five- toed mammals. 



" In the human hand there are fifteen muscles which have special 

 functions in the complicated movements of the organ. Onlj' five^ 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." 



^ Others have been discovered in later dissections. 



