THE TARSUS 



105 



much the hirger, and is not very well adapted to the inner lip of the trochlea; in 

 the fresh state, however, it is completed and rendered more congruent by the 

 curved accessory fibro-cartilage. 



The lateral borders converge to the apex below, and 

 each forms a prominence or angle at the base. The inner 

 angle and the adjacent part of the border give attach- 

 ment to the fibro-cartilage of the patella, which increases 

 the articular surface. The external border is rounded 

 and its angle is less prominent. 



The base (Basis patellae) faces upward and back- 

 ward, and is convex transversely, concave from before 

 backward. 



The apex forms a l)lunt point direcced downward. 



Development. — The patella develops as a sesamoid 

 bone from a single center in a cartilaginous deposit in 

 the tendon of the cjuadriceps femoris nmscle. 



THE TARSUS 



The tarsus or hock of the horse usually comprises 

 six short bones (^Ossa tarsi), but exceptionall}' seven are 

 present. 



The Tibial Tarsal Bone 



The tibial tarsal bone (Os tarsi tibiale, astragalus, 

 or talus) is the inner bone of the proximal row. It is 

 extremely irregular in form, but may be considered as 

 offering six surfaces for description. 



The superior and the anterior or dorsal surface are 

 continuous, and form a trochlea for articulation with the 

 distal end of the tibia. The trochlea consists of two 

 oblique ridges with a deep groove between them, which 

 curve spirally forward, downward, and outward. There 

 is usually a shallow synovial fossa in the groove. The 

 inferior surface is convex from before backward, and 

 most of it articulates with the central tarsal; externally 

 it has an oblique facet for the fourth tarsal, and a non- 

 articular groove cuts into the surface to its middle. The posterior or plantar 

 surface is oblique and extremely irregular; it presents four facets for articulation 

 with the fibular tarsal bone ; the facets are separated by rough excavated areas, and 

 the largest fossa (Sulcus tali) forms with a corresponding one on the fibular 



72. — Frontal Sictiox of 

 Right Tibia of Horse, 

 Anterior View. 



Base 



Accessory 

 cartilage 



Base 

 Accessor ij 

 cartilage 



Fig. 73. — Right Patella of Horse, Anterior View. Fig. 74.— Right Patella of Horse, Posterior View. 



tarsal a cavity termed the sinus tarsi. The internal surface bears on its lower part 

 a large tuberosity, and on its upper part a small one for the attachment of the 



