From the Greek diafragma, the diaphragm
is the transverse muscular partition below the heart and lungs and above
the liver, stomach, and rest of the intestinal canal, fully developed in
Mammals only. In birds it is incomplete and rather differently arranged,
(a) of the pulmonary or transverse, and...
The first arises from the second to the sixth pairs
of ribs near the lateral edge of the lungs, and spreads over their ventral
surface as an aponeurotic membrane, while it is connected with the vertebral
column as the median vertical septum; completely separating the lungs and
the cervical air-sacs from the rest of the thoraco-abdominal cavity. Small
voluntary muscles arising from the ribs and from the sternum extend over
part of the aponeurosis.
(b) of the abdominal or oblique portion.
The second or oblique half is entirely membranous
without muscular fibres: it forms the continuation of the ventral margin
of the vertical median septum, and is connected with the pericardium and
with the medio-ventral portion of the sternum, while the rest extends obliquely
through the abdominal cavity to the posterior and ventral margins of the
sternum. The space thus enclosed is the subpulmonary chamber, divided into
a right and a left half by the vertical septum. Three transverse septa
divide again either half into four loculi, into each of which one of the
three or four post-bronchial Air
Sacs extends from the lungs.
Consequently the whole of the diaphragmatic membranes
divide the entire thoraco-abdominal cavity into three chambers:
The Pulmonary chamber, anteriorly and dorsally from
the pulmonary septum, containing the lungs and cervical air-sacs.
The Sub- pulmonary chamber, anteriorly and ventrally
from the oblique septum, and ventrally from the pulmonary septum, containing
most of the air-sacs.
The Cardio-abdominal chamber, posteriorly from or
below the oblique septum, containing the heart and the rest of the intestines.