320
UNIT TWO
The
transversus abdominis
(trans-ver
sus ab-dom
ı
˘-nis)
forms a third layer of muscle beneath the external and inter-
nal obliques. Its F
bers run horizontally from the lower ribs,
lumbar vertebrae, and ilium to the linea alba and pubic
bones. It functions in the same manner as the external and
internal obliques.
The
rectus abdominis
(rek
tus ab-dom
ı˘-nis) is a long,
straplike muscle that connects the pubic bones to the ribs
and sternum. Three or more F
brous bands cross the muscle
transversely, giving it a segmented appearance. The muscle
functions with other abdominal wall muscles to compress
the contents of the abdominal cavity, and it also helps to fl
ex
the vertebral column.
Muscles of the Pelvic Outlet
Two muscular sheets span the outlet of the pelvis—a
deeper
pelvic diaphragm
and a more superF
cial
urogeni-
tal diaphragm.
The pelvic diaphragm forms the fl
oor of the
pelvic cavity, and the urogenital diaphragm F
lls the space
within the pubic arch.
Figure 9.36
and
table 9.11
show the
muscles of the male and female pelvic outlets. They include
the following:
Pelvic Diaphragm
Urogenital Diaphragm
Levator ani
SuperF
cial transversus perinei
Coccygeus
Bulbospongiosus
Ischiocavernosus
Sphincter urethrae
Pelvic Diaphragm
The
levator ani
(le-va
tor ah-ni
) muscles form a thin sheet
across the pelvic outlet. They are connected at the midline
posteriorly by a ligament that extends from the tip of the
coccyx to the anal canal. Anteriorly, they are separated in
the male by the urethra and the anal canal, and in the female
by the urethra, vagina, and anal canal. These muscles help
support the pelvic viscera and provide sphincterlike action
in the anal canal and vagina.
An
external anal sphincter
under voluntary control and an
internal anal sphincter
formed of involuntary muscle F bers of
the intestine encircle the anal canal and keep it closed.
The
coccygeus
(kok-sij
e-us) is a fan-shaped muscle that
extends from the ischial spine to the coccyx and sacrum. It
aids the levator ani.
Urogenital Diaphragm
The
superF
cial transversus perinei
(soo
per-F sh
al trans-
ver
sus per
ı˘-ne
i) consists of a small bundle of muscle F
bers
that passes medially from the ischial tuberosity along the
posterior border of the urogenital diaphragm. It assists other
muscles in supporting the pelvic viscera.
In males, the
bulbospongiosus
(bul
bo-spon
je-o
sus)
muscles are united surrounding the base of the penis. They
assist in emptying the urethra. In females, these muscles are
separated medially by the vagina and constrict the vaginal
opening. They can also retard the fl ow of blood in veins,
Extensors
The
extensor carpi radialis longus
(eks-ten
sor kar-pi
ra
de-
a
lis long
gus) runs along the lateral side of the forearm, con-
necting the humerus to the hand. It extends the wrist and
assists in abducting the hand (see F
gs. 9.33 and 9.34).
The
extensor carpi radialis brevis
(eks-ten
sor kar-pi
ra
de-a
lis brev
ı
˘s) is a companion of the extensor carpi radia-
lis longus and is located medially to it. This muscle runs from
the humerus to metacarpal bones and extends the wrist. It
also assists in abducting the hand (see F gs. 9.33 and 9.34).
The
extensor carpi ulnaris
(eks-ten
sor kar-pi
ul-na
ris)
is located along the posterior surface of the ulna and con-
nects the humerus to the hand. It extends the wrist and
assists in adducting the hand (see F
gs. 9.33 and 9.34).
The
extensor digitorum
(eks-ten
sor dij
ı
˘-to
rum)
runs medially along the back of the forearm. It connects
the humerus to the posterior surface of the phalanges and
extends the F
ngers (see F
gs. 9.33 and 9.34).
A structure called the
extensor retinaculum
consists of
a group of heavy connective tissue F bers in the fascia of the
wrist (see F g. 9.33). It connects the lateral margin of the radius
with the medial border of the styloid process of the ulna and
certain bones of the wrist. The retinaculum gives off branches
of connective tissue to the underlying wrist bones, creating a
series of sheathlike compartments through which the tendons
of the extensor muscles pass to the wrist and F ngers.
Muscles of the Abdominal Wall
The walls of the chest and pelvic regions are supported directly
by bone, but those of the abdomen are not. Instead, the ante-
rior and lateral walls of the abdomen are composed of layers of
broad, fl attened muscles. These muscles connect the rib cage
and vertebral column to the pelvic girdle. A band of tough con-
nective tissue, called the
linea alba
(lin
e-ah al
bah), extends
from the xiphoid process of the sternum to the symphysis pubis.
It is an attachment for some of the abdominal wall muscles.
Contraction of these muscles decreases the volume of the
abdominal cavity and increases the pressure inside. This action
helps force air out of the lungs during forceful exhalation and
also aids in defecation, urination, vomiting, and childbirth.
The abdominal wall muscles are shown in
f gure 9.35
,
reference plate 67, and are listed in
table 9.10
. They include
the following:
External oblique
Transversus abdominis
Internal oblique
Rectus abdominis
The
external oblique
(eks-ter
nal o
˘-ble
¯k) is a broad, thin
sheet of muscle whose F
bers slant downward from the lower
ribs to the pelvic girdle and the linea alba. When this muscle
contracts, it tenses the abdominal wall and compresses the
contents of the abdominal cavity.
Similarly, the
internal oblique
(in-ter
nal o
˘-ble
¯k) is a broad,
thin sheet of muscle located beneath the external oblique. Its
F bers run up and forward from the pelvic girdle to the lower
ribs. Its function is similar to that of the external oblique.
previous page 350 David Shier Hole's Human Anatomy and Physiology 2010 read online next page 352 David Shier Hole's Human Anatomy and Physiology 2010 read online Home Toggle text on/off