118 



THE SKELETON OF THE HORSE 



The distal extremity is fused with the tibia, constituting the lateral malleolus. 



Development. — This resembles that of the ulna. The embrj^onic cartilaginous 

 fibula extends the entire length of the leg, but does not articulate with the femur. 

 The distal part of the shaft is usually reduced to a fibrous band. Three centers of 

 ossification appear, one each for the shaft and the extremities. The distal end 

 unites early with the tibia, forming the lateral malleolus. 



It is interesting to note that in some cases the entire shaft of the fibula develops, a reversion 

 to the condition in the IMiocene ancestors of the horse. 



THE PATELLA 



The patella is a large sesamoid bone which articulates with the trochlea of the 

 femur. It presents for description two surfaces, two borders, a base, and an apex. 



The anterior, free surface (Facies libera) is quadrilateral, convex, and rough 

 for muscular and ligamentous attachment. 



The articular surface (Facies articularis) is also quadrilateral, but much less 

 extensive. It presents a vertical rounded ridge, which corresponds to the groove 



.1 pi'X 



Fig. 100. — Right Patella of Horse; Anterior View. 

 1, Attachnient area of middle patellar ligament; 

 2, attachment area of lateral patellar ligament and 

 biceps femoris. 



Accessory cartilage Apex 



Fig. 101. — Right Patella of Horse; Posterior View. 

 1, Medial part, 2, lateral part, of articular surface. 



on the trochlea of the femur, and separates two concave areas. Of the latter, the 

 medial one is much the larger, and is not verj^ well adapted to the corresponding 

 ridge of the trochlea; in the fresh state, however, it is completed and rendered more 

 congruent Ijy the curved accessory fibro-cartilage. 



The borders, medial and lateral, converge to the apex below, and each forms an 

 angle at the base. The medial border is concave. The lateral border is rounded 

 and its angle is less prominent. The medial angle and the adjacent part of the 

 posterior margin of the base give attachment to the fibro-cartilage of the patella 

 (Fibrocartilago patellae). 



The base (Basis patellse) faces upward and backward, and is convex trans- 

 versely, concave from before backward. 



The apex (Apex patelliS') forms a blunt point directed distally. 



Development. — The patella develops as a sesamoid bone from a single center 

 in a cartilaginous deposit in the tendon of the quadriceps femoris muscle. 



THE TARSUS 



The tarsus or hock of the horse usually comprises six short bones (Ossa tarsi), 

 but exceptionally seven are present. 



