The cervical canal communicates with the
uterine cavity via the
internal orifice of the uterus (or
internal os) and with the
vagina via the
external orifice of the uterus (
ostium of uterus or
external os). The internal orifice of the uterus is an interior narrowing of the uterine cavity. It corresponds to a slight constriction known as the
isthmus that can be seen on the surface of the
uterus about midway between the apex and base. The external orifice of the uterus is a small, depressed, somewhat circular opening on the rounded extremity of the
cervix, opening to the vagina. Through this aperture, the cervical cavity communicates with that of the vagina. The external orifice is bounded by two lips, an anterior and a posterior. The anterior is shorter and thicker, though it projects lower than the posterior because of the slope of the cervix. Normally, both lips are in contact with the posterior vaginal wall. Prior to pregnancy, the external orifice has a rounded shape when viewed through the vaginal canal (as through a
speculum). Following
childbirth, the orifice takes on an appearance more like a transverse slit or is "H-shaped". The wall of the canal presents an anterior and a posterior longitudinal ridge, from each of which proceed a number of small oblique columns, the
palmate folds, giving the appearance of branches from the stem of a tree; to this arrangement the name
arbor vitae uteri is applied. The folds on the two walls are not exactly opposed, but fit between one another so as to close the cervical canal.
Histology The cervical canal is generally lined by "endocervical mucosa" which consists of a single layer of mucinous columnar epithelium. However, after menopause, the functional
squamocolumnar junction moves into the cervical canal, and hence the distal part of the cervical canal may be lined by stratified squamous epithelium (conforming to a "type 3 transformation zone"). ==Pathology==