The abdominal cavity (Cavum abdominis) is the largest of the body
cavities. It is separated from the thoracic cavity by the diaphragm and
is continuous behind with the pelvic cavity.
It is ovoid in form but somewhat compressed laterally. Its long axis
extends obliquely from the center of the pelvic inlet to the sternal
part of the diaphragm. Its dorso-ventral diameter is greatest at the
first lumbar vertebra, while its greatest transverse diameter is a
little nearer the pelvis.
The =dorsal wall= or roof is formed by the lumbar vertebræ, the lumbar
muscles, and the lumbar part of the diaphragm.
The =lateral walls= are formed by the oblique and transverse abdominal
muscles, the abdominal tunic, the anterior parts of the ilia, the
cartilages of the asternal ribs, and the parts of the posterior ribs
which are below the attachment of the diaphragm.
The =ventral wall= or floor consists of the two recti, the aponeuroses
of the oblique and transverse muscles, the abdominal tunic, and the
xiphoid cartilage.
The =anterior wall= is formed by the diaphragm, which is very deeply
concave, thus greatly increasing the size of the abdomen at the expense
of the thorax.
It should be noted that the diaphragm also concurs practically in the
formation of a considerable part of the lateral walls, since its
costal portion even during ordinary inspiration lies directly on the
ribs over a width of four or five inches (ca. 10 to 12 cm.); in
expiration this area of contact would be about twice as wide,
including about all of the fleshy rim. This fact is of clinical
importance, with reference to auscultation and percussion, and
penetrating wounds. The cupola of the diaphragm extends as far forward
as a plane through the sixth intercostal space to the right of the
heart.
There is no wall between the abdominal and pelvic cavities. The line of
demarcation here is the =terminal line= (Linea terminalis) or brim of
the pelvis; it is formed by the base of the sacrum, the ilio-pectineal
lines, and the anterior borders of the pubic bones.
The muscular walls are lined by a layer of fascia, distinguished in
different parts as: (1) the diaphragmatic fascia; (2) the transversalis
fascia; (3) the iliac fascia; (4) the deep layer of the lumbo-dorsal
fascia.
The subperitoneal or extraperitoneal connective tissue (Tela subserosa)
unites the fascia and peritoneum. It is composed of areolar tissue, more
or less loaded with fat according to the condition of the subject,
except over the diaphragm. It sends laminæ into the various peritoneal
folds.
The peritoneum, the serous membrane which lines the cavity, will be
described later.
The abdominal walls are pierced in the adult by =five apertures=. These
are: the three openings in the diaphragm which transmit the aorta,
posterior vena cava, and the œsophagus; the inguinal canals, which
contain the spermatic cord or the round ligament (in female carnivora).
In the fœtus there is the umbilical opening also.
The cavity contains the greater part of the digestive and urinary
organs, part of the internal generative organs, numerous nerves,
blood-vessels, lymph vessels and glands, ductless glands (spleen and
adrenal bodies), and certain fœtal remains.
For topographic purposes the abdomen is divided into nine regions by
imaginary planes.[88] Two of these planes are sagittal, and two are
transverse. The sagittal planes cut the middles of the inguinal
(Poupart’s) ligaments; the transverse planes pass through the last
thoracic and fifth lumbar vertebræ, or the lower end of the fifteenth
rib and the external angle of the ilium respectively. The transverse
planes divide the abdomen into three zones, one behind the other, viz.,
=epigastric=, =mesogastric=, and =hypogastric=: these are subdivided by
the sagittal planes as indicated in the subjoined table.
Left parachondriac Xiphoid Right parachondriac
Left lumbar Umbilical Right lumbar
Left iliac Prepubic Right iliac.
Other useful regional terms are: sublumbar, diaphragmatic, inguinal. The
first two require no explanation. The inguinal regions (right and left)
lie in front of the inguinal (Poupart’s) ligament. The flank is that
part of the lateral wall which is formed only of soft structures. The
depression on its upper part is termed the paralumbar fossa.