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= or =free surface= (Facies libera) is irregularly
quadrilateral, convex, and rough for muscular and ligamentous
attachment.
The =posterior= or =articular surface= is smaller and is triangular in
outline. It presents a vertical rounded ridge, which corresponds to the
groove on the trochlea of the femur, and separates two concave areas. Of
the latter, the inner cavity is much the larger, 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=.
[Illustration:
FIG. 72.—FRONTAL SECTION OF RIGHT TIBIA OF HORSE, ANTERIOR VIEW.
]
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 attachment 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 patellæ) faces upward and backward, and is convex
transversely, concave from before backward.
The =apex= forms a blunt point directed downward.
=Development.=—The patella develops as a sesamoid bone from a single
center in a cartilaginous deposit in the tendon of the quadriceps
femoris muscle.