cervix

The narrow lower part of the uterus, with an opening connecting the uterus to the vagina.

Reach as far back as you can into your vagina and you may be able to feel your cervix, the lower part of your uterus. How can you tell? If you press on it gently, you may find that it feels like an enlarged version of the tip of your nose, perhaps with a small dimple in the center. It's pretty firm, but can be moved around a little.

The dimple you may feel in the center of your cervix is the opening of the cervix, a.k.a. the os. ("Os" rhymes with "floss," in case you were curious or reading out loud or something.) Menstrual flow passes out of the uterus through the os, down the vaginal canal and out the vaginal opening. Traveling in the other direction, sperm enter the uterus through the os to fertilize the egg. And, although the os is usually very tiny, it can dilate wide enough to permit a baby to pass through. As you can see (and/or feel), the cervix pretty much closes off the upper end of the vagina. That's why you don't have to worry that tampons or other objects will get lost inside you. They can't. They've got nowhere to go but back out, through your vaginal opening.

Referrence:

The round cervix is the neck of the uterus that projects into the vagina.

The cervix is a firm structure, composed of a connective tissue matrix of glands and muscular tissue elements, that becomes soft and pliable under the influence of hormones produced during pregnancy.

Glandular tissue provides a rich supply of protective mucus that changes in character and quantity during the menstrual cycle and during pregnancy.

The cervix is richly supplied with blood from the uterine artery and can be a site of significant blood loss during delivery.

The opening of the cervix, the os, forms a pathway between the uterus and the vagina.

The vaginal opening is called the external os and the uterine opening, the internal os.

The space between these two openings is the endocervical canal.

Secretions from the columnar epithelium of the endocervix protect the uterus from infection, alter receptivity to sperm, and form a mucoid "plug" during pregnancy.

The endocervical canal provides a route for menstrual discharge and sperm entrance.

CERVICAL MUCUS

Cervical mucus is a complex, heterogeneous secretion produced by the glands of the endocervix.

It is composed of 92% to 98% water and 1% inorganic salts, mainly sodium chloride.

The mucus also contains simple sugars, polysaccharides, proteins, and glycoproteins.

Its pH usually is alkaline, ranging from 6.5 to 9.0.

Its characteristics are strongly influenced by serum levels of estrogen and progesterone.

Estrogen stimulates the production of large amounts of clear, watery mucus through which sperm can penetrate most easily.

Progesterone, even in the presence of estrogen, reduces the secretion of mucus.

During the luteal phase of the menstrual cycle, mucus is scant, viscous, and cellular.

Two methods are used to examine the properties of cervical mucus and correlate them with hormonal activity.

Spinnbarkeit is the property that allows cervical mucus to be stretched or drawn into a thread.

Spinnbarkeit can be estimated by stretching a sample of cervical mucus between two glass slides and measuring the maximum length of the thread before it breaks.

At midcycle, spinnbarkeit usually exceeds 10 cm.

A second method of estimating hormonal levels is ferning, or arborization.

Ferning refers to the characteristic microscopic pattern that results from the crystallization of the inorganic salts in the cervical mucus when it is dried.

As the estrogen levels increase, the composition of the cervical mucus changes, so that dried mucus begins to demonstrate ferning in the later part of the follicular phase.

The absence of ferning can indicate inadequate estrogen stimulation of the endocervical glands or inhibition of the endocervical glands by increased secretion of progesterone.

Persistent ferning throughout the menstrual cycle suggests anovulatory cycles or insufficient progesterone secretion.