LOCOMOTOR SYSTEM 609 



words, the corona and suffraginis are immovably locked, and 

 for the time being the joint between the fetlock and foot is 

 obliterated. As the locking is entirely ligamentous, the strain 

 must be considerable. When the foot comes to the ground, con- 

 cussion to the pastern-joint is saved by the fibrous plate which 

 exists at its posterior part. This plate is on the corona, and is 

 hollowed out for the reception of the rounded surface of the 

 suffraginis. 



The Pedal- Joint is formed by three bones — the second and 

 third phalangeal and the navicular bone. The chief movements 

 occurring are flexion and extension. This joint is the centre 

 of the entire movement of the leg ; the body rotates over it. 

 Relatively it is a small articujatory surface built up and 

 strengthened by the hoof, the cartilages of the foot, the flexor 

 and extensor tendons of the foot, and the connecting ligaments 

 peculiar to the bones forming the joint. It is a yielding or 

 elastic joint, due to the introduction of the navicular bone into 

 its posterior part ; this functions as a sesamoid, and by yielding 

 under pressure counteracts shock. The pedal-joint is so deeply 

 buried within the foot, and strengthened by the various structures 

 named, that though it is the centre of movement during the 

 passage of the body over the limb, it is practically immune 

 against the diseases of locomotion, so common in other joints. 

 Not so the joint last considered — the pastern ; it is frequently 

 the seat of trouble, probably due to the strain on the ligaments 

 when the bones become locked during extension. 



Joint-Locking. — Repeated attention has been drawn to joint- 

 locking, and the example last given of the mechanism enables 

 a more comprehensive statement to be made. The whole of the 

 joints in the fore-leg of the horse are locked during extension, 

 from knee to coronet. The only joints where movement is 

 occurring are the shoulder, elbow, and foot (the articulation 

 between the corona and pedis) . The locking occurs when the limb 

 on the ground is no longer vertical — when, in fact, it is assuming 

 the position shown by the fore-leg in Fig. 193 ; from that moment 

 every degree of further extension locks knee, fetlock, and pastern 

 the tighter. This renders the limb from the arm to the foot as 

 rigid as if it were composed of one piece of bone. It also ex- 

 plains why the posterior part of the knee-joint is one mass of 

 interlacing ligamentous structure. 



The Hip- Joint has a wide range of movement in the direc- 

 tion of flexion, extension, and rotation ; but owing to the position 

 of the round ligament (ligamentum teres) , outward movement at 

 right angles to the body is limited. The pubio-femoral ligament, 

 which is confined to the horse, is derived from the pre pubic 

 jtendon of the abdominal muscles. It is far from clear why a 



39 



