41

21

sides of the last costal vertebra : these crura are almost entirely tendious ; they expand
as they advance forwards, and distribute their aponeurotic fibres in a manner remarkably
analogous to the disposition of the fleshy fibres of the lesser muscle of the diaphragm in
Mammalia. The mesial fibres decussate in front of the aorta : the lateral ones arch
outwards ; the rest diverge, to constitute the great central tendon. Here they cross
each other in various directions and form distinct and regular decussations around the
orifices through which the coeliac artery, with the anterior splanchnic nerve, (Pl. VI. l,)
and the mesenteric artery and nerves (Pl. VI. m,), pass into the abdomen : the most
notable decussation is formed by two broad bands, immediately behind the large oeso-
phageal aperture, which is separated only by a very narrow transverse chord from the
anterior fissure through which the pericardium protrudes, and the inferior vena cava
passes : the two broad decussating bands expand, to form the anterior boundary of the
diaphragm, and are inserted into the lateral processes of the sternum.

The muscular or costal part of the diaphragm is formed , as in the Ostrich, by a num-
ber of separate, broad, and thin fasciculi, which come off from the third, fourth, fifth,
sixth, and seventh vertebral ribs, near their junction with the sternal ones : these fas-
ciculi expand, and are gradually lost upon the dorsal surface of the aponeurotic part of
the diaphragm, but do not form a continuous expanse of muscle, nor constitute the en-
tire thickness or substance of the diaphragm at any point : they are, consequently , in-
visible on the abdominal side of the diaphragm ; and the aponcurosis of the diaphragm,
together with the almost aponeurotic cellular layer of the peritoneum, with which it is
continuous, requires to be reflected inwards, as at Pl.VI. B.B. fig. 1., to bring the digi-
tations representing the great muscle of the diaphragm into view.

The existence of a diaphragm in a rudimental condition in birds has long been recog-
nized. Hunter left a beautiful figure of the costal portion of the diaphragm in the Ostrich,
which has been published in the second volume of the Catalogue of his Physiological
Collection, Pl. XXXVI. In this, as well as in the other large Struthious birds, there is
also a pars vertebralis or analogue of the lesser muscle of the diaphragm, which rises by
two tendinous crura from the last dorsal vertera, and in the Emeu by a double origin
on each side. Nevertheless their diaphragm is incomplete ; first , by reason of an arrest
of its centripetal development, which leaves a permanent defect of union in the mesial
plane ; and secondly, by the large perforations for the abdominal air-cells.

The mechanism of respiration in the Apteryx is essentially the same as in other birds ;
and a more muscular diaphragm than it possesses would be unnecessary as a part of that
mechanism. The abdominal surface of the diaphragm, as in the Mammalia, is princi-
pally in contact with the liver, spleen, and stomach ; but its thoracic surface, as we have
already seen, does not support the heart, and it is separated from the lungs by the in-
terposition of a series of small but well-marked air-cells. There is no thoracic serous
sac or pleura.

Thus, although the respiratory organs are confined to the chest, and the Apteryx offers