The ovarian surface is largely covered by peritoneum
except at the attached border where nerves and vessels
enter. The relationship between ovarian cortical and
medullary areas is unusual in the mare (see Figure 1-4 ).
The ovary of the mare is “inside-out” compared with
those of other farm animal species. In other words,
the medullary or vascular zone is superfi cial and the cortical zone (which contains the follicles) is in the interior
of the gland. The cortical tissue reaches the surface
only at the depression of the ventral or free border. This,
therefore, is the only area from which normal ovulation
occurs and is appropriately termed the ovulation fossa.
The ovulation fossa is covered by a layer of short
polygonal cells, which are a remnant of the primitive
germinal epithelium. The ovulation papilla of the corpus
luteum does not project from the convex surface of
the ovary as it does in other species but rather protrudes
into the ovulation fossa.