170 TJSE ARTICULATIONS. 



a fibro-cartilaginous thickening, on which glides the perforans tendon. _ It 

 is attached, above, to the tibia, below, to the astragalus and calois ; at its sides 

 it is mixed with the two superficial lateral ligaments, and the astragalian 

 fasciculus of the middle internal ligament. Its internal face is lined by- 

 articular synovial membrane ; the external is covered and lubricated by the 

 vaginal serous membrane which facilitates the gliding of the perforans 

 tendon in the tarsal sheath. 



Synovial membrane. — This membrane is developed at the internal face 

 of the two capsular ligaments, nearly covers the three internal ligaments, 

 and lines the external deep ligament. It communicates, in front and below, 

 with the synovial membrane proper to the articulation of the two rows of 

 tarsal bones. When it becomes the seat of dropsical effusion, it is always 

 distended forwards and inwards, because it is only sustained at that place 

 by the anterior capsular ligament. But the effusion may also raise the 

 posterior ligament and produce hernia in the hollow of the hock, behind the 

 lateral ligaments. It is not, therefore, absolutely correct to attribute all the 

 synovial tumours in the hollow of the hock to dilatation of the tarsal 

 tendinous sheath. 



Movements. — Nothing can be less complicated than the mechanism of the 

 tibio-tarsal articulation ; this joint only permitting two opposite movements, 

 those of flexion and extension, which are so simple and precise that we may 

 dispense with a description of the manner in which they are executed. It 

 may only be remarked that, in order to prevent contact between the leg 

 and foot during flexion, the latter fraction of the limb deviates a little 

 outwards, owing to the marked obliquity of the articular grooves. 



Articulation of the Bones of the First Eow, oe Oaloaneo- 

 ASTRAGALOiD ARTICULATION. — This is a compouud arthrodial joint, resulting 

 from the coaptation of the three or four articular facets of the posterior face 

 of the astragalus with the analogous facets of the calcis. 



This joint is maintained by the lateral ligaments of the tibio-tarsal 

 articulation, and by four calcaneo-astragaloid ligaments — a superior, external, 

 internal, and the last interosseous. 



The superior calcaneo-astragaloid ligament is formed of short parallel 

 fibres thrown across from one bone to the other, and is situated towards the 

 superior extremity of the pulley of the astragalus ; it is lined superiorly by 

 the synovial membrane of the tibio-tarsal articulation. 



The lateral ligaments are two very thin fasciculi concealed by the 

 ligaments which bind, laterally, the tibia to the tarsal bone. 



The interosseous ligament is very strong, and occupies a great portion of 

 the rugged excavation which separates the articular facets. 



This articulation does not usually possess proper synovial capsules. 

 Two prolongations of the synovial membrane of the two rows, in ascending 

 between the calcis and astragalus, facilitate the gliding of the two inferior 

 facets. An analogous prolongation of the tibio-tarsal synovial membrane is 

 effected for the superior facets, and it is not rare to find this prolongation 

 form a distinct capsule. 



Movements nearly null 



Articulation op thk Bones of the Second Eow with each other. — 

 These bones, four in number, are brought into contact in the following 

 manner : — The cuboides responds to the scaphoid by two facets, one anterior, 

 the other posterior ; it articulates with the great cuneiform by two similar 

 facets, the posterior of which is not always present. The scaphoid is united 

 to the two cuneiforms by the large convex facet occupying its entire lower 



