50 THE ANATOMY OF THE HOESE. 



The Intee-metacabpal Articulations. The head of the large meta- 

 carpal bone responds to one of the small metacarpals on each side by a 

 small synovial joint lubricated by a process from the carpo-metacarpal 

 synovial membrane. Below that point the union of the bones is main- 

 tained by short interosseous fibres, which, in adult animals, are very 

 frequently ossified. The lower extremities of the splint bones, however, 

 for a short distance above the little knob that terminates them, remain 

 freely movable, as may be felt by manipulation in the living animal. 

 In addition to the interosseous fibres, the ligaments of the carpus which 

 get inserted in common into the heads of both large and small metacarpal 

 bones, contribute to the union of these bones. 



THE PETLOCK-JOINT (pLATB 11, FIGS. 4-7). 



This, which is technically termed the metacarpo-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 SuPBEiOB Sesamoidean or Susfensoey Ligament. — The main por- 

 tion of this ligament is lodged in the channel formed by the three meta- 

 carpal bones, where it is related by its posterior face to the perforans 

 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 metatarsus). These two portions blend, and descend be- 

 hind 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 ou 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 admix- 

 ture of striped muscular tissue. The presence of muscular tissue here 

 points to the conclusion (strengthened by other considerations) that the 



