76 TOPOGEAPHICAL ANATOMY OF 



closely united to the third metacarpal by interosseous ligaments 

 (ligamenta interossea), the short fibres of wliieh generally become 

 ossified as age advances. 



The METACAKPOPiiALANGEAL ARTICULATION (articulatio metacarpo- 

 phalangea). — The head of the third metacarpal bone possesses an 

 articular surface composed of two convexities separated by an antero- 

 posterior ridge. These three parts together form a continuous smooth 

 area the width of which increases towards the back (volar part) of the 

 joint. The proximal end of the first phalanx is closely moulded upon 

 the head of the metacarpal bone. At the back of the joint the two 

 sesamoid bones (of the first phalanx) together form a continuation of 

 the articular surface of the first phalanx ; but it should be remembered 

 that, in the horse, there is no direct contact between the sesamoids and 

 the phalanx. 



The joi7it capsule (capsula articularis) in front of the joint is supported 

 by and connected with the tendons of the digital extensors. On the 

 flexor aspect of the joint it is attached to the articular margins of the 

 third metacarpal and sesamoid bones, and has a pocket-like extension 

 insinuated between the modified interosseous muscle and the third 

 metacarpal bone. If the capsule becomes pathologically distended, 

 the support afforded by tendons and ligaments is so disposed that a 

 bulging can only take place through the interval between the two 

 limbs of the interosseous muscle. From the circumstance that the 

 flexor tendons lie superficial to this interval, it follows that more 

 marked distension must of necessity make itself manifest on each side 

 of the limb, between the flexor tendons and the interosseous muscle 

 immediately proximal to the sesamoid bones. 



The lateral and medial collateral liga,ments (ligamentum collaterale 

 laterale : mediale) are stout bands springing from the depressions at the 

 side of the head of the third metacarpal bone. Each ligament is com- 

 posed of superficial fibres attached close to the articular margin of 

 the first phalanx, and a deep and stronger set of fibres inserted 

 to the sesamoid bones and the angular projection at the base of 

 the phalanx. 



On cleaning the surface of the collateral ligaments, the dissector 

 will observe that a sheet of oblique fascial fibres covers them and the 

 joint capsule on the dorsal face of the joint. 



An exceedingly dense inter-sesamoid'^ ligament (ligamentum inter- 

 sesamoideum) binds the two sesamoid bones together, and extends 



1 Semmmn [L.], (T;'j(Tajxov (sesamon) [Gr.], sesame (the plants Sesamum indicum 

 and S. orientale carry oil-yielding seeds). etSos (eidos) [Gr.], form. The sesamoid 

 bones of the smaller mammals resemble sesame seeds. 



