THE ABDOMINAL CAVITY. 995 
The superficial epithelial cells are ciliated and somewhat cylindrical between the 
fourth and eighth months of foetal life (Neumann) ; subsequently the adult condition is 
established. 
THE ABDOMINAL CAVITY. 
As the remaining parts of the digestive system lie within the abdomen it will 
be necessary to describe that cavity as a whole, and to refer briefly to its linine 
membrane—the peritoneum—before passing on to the consideration of the viscera 
which are contained within it. 
The abdomen is that portion of the cavity of the trunk which lies below the 
diaphragm. It is the largest of all the cavities of the body, and contains the 
greater part of the digestive, urinary, and generative systems of organs, in addition 
to numerous vessels, nerves, and other structures. 
Shape.—In general, the abdominal cavity is of a somewhat oval form, with the 
long axis directed vertically, and the wide end upwards. It is strongly flattened 
from before backwards, and is encroached upon in the middle line posteriorly by 
the projection forwards of the vertebral column, on each side of which it presents 
the appearance of a deep wide groove. 
Boundaries.—The cavity is limited above by the concave vault of the diaphragm, 
which is dome-shaped and divided into a right and a left cupola by an interven- 
ing depression. Into the right cupola fits the greater part of the liver; in the left 
hie the stomach and spleen. On the upper surface of each cupola is placed the base 
of the corresponding lung, whilst between them, on the depression, rests the under 
surface of the heart. 
During expiration, the right cupola ascends almost to the level of the nght 
nipple ; it is highest at a point about one inch internal to the nipple line, and here 
it reaches the upper border of the fifth rib, or even the middle of the fourth inter- 
costal space. On the left side it is one-half to one inch (12-25 mm.) lower, and 
in the middle line it crosses the inferior extremity of the gladiolus about the 
level of the sixth nb cartilage. 
Below, the cavity is limited by the pelvic floor, formed by the levatores ani, 
and coecygel muscles, covered on their upper surface by the pelvic fascia. The 
anterior wall is formed by the aponeuroses of the three flat abdominal muscles, 
together with the two recti, which latter constitute powerful braces for the 
wall, on each side of the middle line. The lateral walls are formed by the muscular 
portions of the obliqui and transversales muscles, and by the iliac bones with the 
iliacus muscles below. 
Finally, the cavity is limited behind by the lumbar portion of the vertebral 
‘column with the psoas muscle on each side, and the quadratus lumborum still further 
out. The iliac bones also take part in the formation of the posterior wall inferiorly. 
The upper portion of the cavity les under cover of the ribs, which afford con- 
‘siderable protection to this part of the abdomen, particularly at the sides and 
behind, in which latter position the cavity is further protected by the vertebral 
column. Anteriorly, on the other hand, the ribs are wanting below the sternum, 
cand here, the abdominal wall is formed only of aponeuroses and muscles. But even 
at the sides and back there is a considerable zone, usually one to two inches wide 
(Cunningham), between the lower ribs above and the crest of the ilium below, 
which has no bony support except that afforded by the vertebral column. 
Whilst the circumference of the diaphragm is attached to the lower part of 
‘the thoracic framework in front and at the sides, and to the lumbar vertebree 
behind, the central portion of the dome, on the other hand, namely, the central 
tendon, is placed high up, under cover of the ribs, and in a more or less horizontal 
plane. As a result, the peripheral muscular part slopes almost vertically upwards 
from the circumference of the thoracic framework to the central tendon, and hes 
for a considerable distance in contact with the deep surface of the ribs; thus the 
diaphragm comes to form, not only the roof of the cavity, but it also enters into 
the formation of the lateral, posterior, and, to a less extent, of the anterior walls ; 
and almost as much of the cavity of the abdomen as of the thorax lies under 
cover of the ribs. 
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