DISSECTION OF THE ANTERIOR LIMB. 51 



THE FETLOCK-JOINT (PLATE 11, FIGS. 4-7). 



This, which is technically termed the metdcarpo-phalangeal articula- 

 tion, is a ginglymoid joint ; and its articular surfaces are furnished by 

 the lower extremity of the large metacarpal bone, the upper extremity 

 of the first phalanx, and the two sesamoid bones. It corresponds to the 

 joint at the knuckles in the human hand. 



Movements. — Flexion and extension are, in the natural state, the only 

 movements executed at the joint ; but by manipulation slight lateral 

 movements may be produced when the joint is fully flexed. In com- 

 plete extension the digit is carried beyond the point at which it lies in a 

 straight line with the metacarpus {over-extension), until the movement is 

 arrested by tension of the suspensory ligament. 



Directions. — The tendons which pass in relation to the joint before 

 and behind having been carefully removed, the ligaments should be 

 dissected and studied in the order of their description. 



The Superior Sesamoidean or Suspensory Ligament. — The main 

 portion of this ligament is lodged in the channel formed by the three 

 metacarpal bones, where it is related by its posterior face to the per- 

 forans tendon and its reinforcing band (subcarpal ligament). It has a 

 double origin behind the carpus, viz., (1) by a superficial layer from the 

 lower row of carpal bones, and (2) by a deeper layer from the upper end 

 of the large metacarpal bone. (In the hind limb it has a similar origin 

 from the tarsus and the metatarsus.) These two portions blend, and 

 descend behind the metacarpus as a flattened band which bifurcates a 

 few inches above the sesamoid bones. Each branch passes to the 

 sesamoid bone of its own side, where a considerable proportion of its 

 fibres become inserted; while the rest is continued in the form of a 

 band which crosses obliquely downwards and forwards over the side of 

 the fetlock to join the extensor tendon on the front of the digit, and be 

 continued with it to the os pedis. The ligament is composed of white 

 fibrous tissue with a constant admixture of striped muscular tissue. The 

 presence of muscular tissue here points to the conclusion (strengthened 

 by other considerations) that the suspensory ligament is a muscle which, 

 in the evolution of the horse, has undergone retrogressive changes, and 

 lost its original function. 1 



The Inferior Sesamoidean Ligaments. These are three in number, 

 and may be distinguished as superficial, middle, and deep. The super- 

 ficial ligament is fixed below to the glenoidal fibro-cartilage developed 

 behind the superior articular surface of the second phalanx. It ascends 

 as a flattened band behind the os suffraginis, where it is placed between 

 the middle ligament and the tendon of the deep flexor; and, widening a 



1 According to Professor D. J. Cunningham {Reports of the Challenger Expedition, 

 Vol. V.), the ligament is the altered flexor brevis of the middle digit, the corresponding 

 muscle in the human subject being the 1st plantar interosseous muscle. 



