Species Variations

13.2.1 Mare (Figure 13.1)

The horse's ovary is spherical in shape, except for a depression on the dorsal aspect. This is the ovulation fossa (Figure 13.1) and is found only in the horse.

6 cm in diameter. The active ovary, during oestrus, is much larger - about 9 cm in diam­

eter. The horse's ovaries are located at the level of the third or fourth lumbar vertebrae,

The ovary of the mare is very large, being the largest in the domestic mammals. A mature but sexually inactive ovary is about in the dorsal abdomen and cranioventral to the tuber coxae.

In all the other domestic animals, and in the human, the follicles are located in the cortex. In the mare they are scattered throughout the depth of the stroma. Mature fol­licles in the horse are very large, being up to 7 cm in diameter and containing 50-80 cc of fluid. Ovulation occurs only in the ovulation fossa; in the other domestic mammals ovulation can occur anywhere on the ovarian surface.

The corpora lutea are also very large in the horse (ovaries of the sheep are almond-shaped and 1.5-2.0 cm in length. They are located at the pelvic inlet close to the external iliac artery. As in the cow the ovaries become more cranial with successive pregnancies. In other respects the ovaries and corpora lutea are similar to those of the cow.

13.2.4 Sow (Figure 13.1)

In both sexually immature and mature pigs the ovary is a regular oval shape, but the surface becomes studded with either knob-like corpora lutea or with follicles that resem­ble a raspberry. The ovary is about 5 cm in length and is located a short distance later- oventral to the pelvic inlet.

Both the follicles and the corpora luteum can be very numerous, corresponding to the size of the litter. It is not unusual for there to be as many as 24 follicles between the two ovaries. After ovulation about 18 corpora lutea may form and be associated with 12 conceptuses. The follicles and the corpora lutea both project from the surface of the ovary and are 0.5-1.0 cm in diameter.

13.2.5 Bitch

The shape of the immature bitch's ovary is regularly oval. When sexually active the ovary becomes lumpy with follicles and corpora lutea. In a long period of inactivity (anoestrus) the old corpora lutea become fibrotic, causing distortion of the ovary. The inactive ovary is 1.0-1.5 cm long in a bitch of about 15 kg. The ovaries are located ven­tral to the third lumbar vertebra and are quite dorsal in the abdomen, associated with short mesovaria and the straight Y-shaped cornua of the uterus (Figure 14.4). The ova­ries are closely anchored to the last one or two ribs by the suspensory ligaments of the ovaries (see Section 3.4.3); these have to be ruptured to enable the surgeon to elevate the ovaries when carrying out an ovariectomy.

The ovarian follicles are multiple and projecting, as in the sow, but are proportion­ately smaller. The corpora lutea are pink in colour.

13.2.6 Queen

The general anatomy of the ovary is similar to that of the bitch, with a proportionate reduction in size. The mesovarium is longer in the cat than in the dog, allowing easier access to the suspensory ligament of the ovary.

13.2 The Ovarian Bursa (Figure 13.1)

The ovarian bursa is a pocket of peritoneum that encloses the ovary; its lumen is con­tinuous with the peritoneal cavity via a slit-like orifice on the medial side. The uterine tube (Fallopian tube or oviduct) perforates the wall of the bursa and opens as the funnel-like infundibulum to receive the ovulated oocytes.

The degree to which the bursa encloses the ovary varies with the species. In the horse the ovary is certainly enclosed, whereas in the ruminants it is hardly enclosed at all. The pig does have a bursa, though not complete, and in the dog the ovary is tightly wrapped within the bursa. Another feature of the bursa in the bitch is that it is heavily loaded with fat, making it difficult to see the ovaries during ovariectomy. In the cat the absence of fat and the elongated mesovarium make surgical removal more straightforward.

13.3