The Cervix Uteri (Figures 14.1-14.4)
The cervix uteri, or neck of the uterus, is the section of the reproductive tract between the body of the uterus and vagina. In all species except the sow it projects somewhat into the cavity of the vagina.
Around this projection there is an annular space (except in the sow) called the vaginal fornix. The cervix comprises a thickened firm wall of dense fibrous tissue and both longitudinal and circular smooth muscle. Internally the cervical canal opens into the body of the uterus via the internal uterine os, and into the vagina via the external uterine os. The canal is tightly closed except during oestrus and parturition. When closed the canal is sealed with a mucus plug.The peritoneum is reflected from the female reproductive organs just caudal to the cervix, making the vagina almost entirely retroperitoneal.
14.5.1 Species variations
Horse: The cervical canal is straight and about 7 cm long. It is lined by longitudinal folds, and the vaginal part forms a rounded projection into the vagina. During copulation semen is introduced into the cervical canal or directly into the uterus. The straight canal and the longitudinal folds enable a catheter to be passed easily in artificial insemination. The collagen of the cervix softens during pregnancy, but contraction of the smooth muscle of the cervix ensures that the canal remains closed until parturition. The cervix can actively relax in a few minutes at parturition.
Ox: The cervix has three or four prominent circular or spiral folds that project caudally (towards the vagina) and tend to interlock with each other. The vaginal part of the cervix is partly fused ventrally with the floor of the vagina so that the vaginal fornix is reduced ventrally but well developed dorsally. The cervical canal is longer than in the mare.
The bull ejaculates into the cranial end of the vagina.
The circular folds of the cervical canal make it more difficult to pass a catheter through the canal than in the mare. During parturition the fibromuscular wall of the cervix softens abruptly, allowing the cervix to be dilated passively by the foetus during birth.Sheep: The cervix of the ewe resembles that of the cow. There are five or six caudally directed circular folds. The vaginal cervix is more prominent than in the cow, consisting of irregular folds projecting further into the vagina and with a slit-like opening.
Ejaculation is into the cranial end of the vagina. Artificial insemination is not usually feasible in the ewe because of the impossibility of rectal guiding of a catheter through the cervical folds.
Pig: The sow's cervix is much longer than in the other species (15-20 cm). In addition to the usual longitudinal folds, there are rows of stud-like projections, each about 1.0 cm in diameter, that interlock with one another. These reduce in size until they hardly exist at either end of the cervix. There is no projection of the vaginal cervix nor a vaginal fornix.
Semen is deposited within the cervical canal, the spiral free part of the penis locking in the canal.
Dog and Cat: In the domestic carnivores the cervix is short and spindle-shaped. There are typical longitudinal folds. Because the cervix slopes caudoventrally, the uterine opening is dorsal and the vaginal opening is ventral; the vaginal fornix is just a small recess on the floor of the vagina.
Semen is usually ejaculated into the cervical canal. The anatomy of the cervix makes catheterisation of the uterus difficult so that semen is deposited around the vaginal cervix in artificial insemination.
14.4