Biomedical Engineering Reference
In-Depth Information
ventrally. The lateral extremes of the perineum are the
ischial tuberosities (
Figure 4.12F
) which in some primates
are covered by ischial calosities. (These are described in
detail at the end of the section “Skin” above.) The perineum
includes all of the soft tissue caudal to the pelvic diaphragm.
The deep perineum of both sexes includes the muscular
external anal sphincter and the urogenital diaphragm which
includes the voluntary sphincter for the urethra
(
Figure 4.12F
). The roots of all the external genitalia are
also found in this region and thus it is traversed by the
continuations of various parts of the genital systems of each
sex. The external genitalia are structurally similar to those
of humans and other mammals. Well-defined columns of
erectile tissue are evident in both the penis and clitoris. The
amount of development, particularly of the roots of the
external genitalia, is species specific. The nerves and
arteries of the area are similar to those of humans and most
other mammals.
colored, particularly during the breeding season (see also
the section “Skin” above). For example, the anal triangle is
bright red and the scrotum bright blue in the adult Eryth-
rocebus patas. The penis of all nonhuman primates is
pendulous, but part of the shaft may be fixed to the ventral
body wall by skin. Considerable species to species varia-
tion exists in the fixation of the penis and in the size and the
side of deviation of the os baculum, as well as many details
of surface features such as penile spines and the size and
shape of the glans. The testes of all adult nonhuman
primates are located in a well-developed scrotum. Descent
of the testes into the scrotum is described in the section on
the male genital system (see the section “Male genital
system” above). The scrotum is pendulous, but it may
contract during the nonbreeding season when the testes
ascend toward the external (superficial) inguinal rings. The
relatively anterior position of the scrotum and the degree of
penile fixation may make the penis appear relatively shorter
in some species than in others. Although usually the
scrotum is dorsal (posterior) to the penis, in a few species it
may appear to be ventral (anterior) to the penis.
Female Perineum
Externally the female perineum incorporates the external
manifestation of the clitoris, the orifice of the urethra, the
vaginal opening, and the anus. Nonhuman primates do not
have long urogenital sinuses. Papillary folds mark the
urethral entrance into the vaginal vestibule. At various
times the vulva may be plugged either by copulatory
seminal plugs or during pregnancy. The size of the labial
folds and clitoris varies tremendously among species. The
most extreme example is in Ateles where the clitoris of the
adult female is more prominent and pendulous than
the penis of the adult male. In this species the clitoris is
often mistaken for a penis by novice observers.
In many species of nonhuman primates the skin of the
female perineum may become swollen, puckered, and
highly colored around the time of ovulation and/or to
a lesser extent in late pregnancy. The swollen area is termed
the sexual skin and plays a role in signaling sexual recep-
tivity. The extent of swelling and color change varies
considerably among species. It may involve extensive areas
from the pubic symphysis to the base of the tail as well as
the adjacent posterior thigh. The amount of fluid accumu-
lated in sexual skin in the perineum is enormous in some
species, e.g. chimpanzees and bonobos (
Deschner et al.,
2004; Paoli et al., 2006
), taking on the appearance of large
fluid-filled pillows which subside naturally following
ovulation. In some species the skin of the face shows
similar but more restrained color changes and occasional
swelling (see also the section “Skin” above).
Clinically Significant Features of Perineal
Morphology
Clinically significant features of perineal morphology
include prolapse of various organs as well as the effects of
puberty and seasonal variation on the appearance of the
perineum.
In both female and male nonhuman primates, puberty
(
Pereira and Altmann, 1985
) can affect the morphology
of the perineum. Prior to puberty the female perineum
remains constant in its appearance. In numerous species at
the time of puberty and during every subsequent ovulatory
cycle the perineum undergoes very distinctive, cyclic
changes.
In the female perineum the morphological changes at
the time of ovulation may include swelling of the sexual
skin and the possible presence of a postcopulatory vaginal
plug. The amount of fluid accumulation in sexual skin in
some species, e.g. Pan and Papio, is so extensive that the
skin may actually rupture. Without outside intervention
these ruptures heal within a few days as tumescence
subsides. Several weeks after ovulation menstrual bleeding
may be evident from the vaginal opening. The length of the
actual cycle varies among species. In species with distinct
breeding seasons, the cyclic changes in the perineum are
evident throughout the breeding season unless the female
becomes pregnant. During the nonbreeding season there are
fewer recognizable changes in the perineum except those
correlated with late pregnancy.
In the male perineum, morphological changes occur
prior to, or at the time of, puberty. The rate of these changes
is species specific depending on when the testes finally
Male Perineum
The male perineum externally includes the anus, scrotum,
and penis. In some species the perineum may be brightly
