40

20

In the extreme case of the raptorial bird above-quoted, the advantage of such an
arrangement appears sufficiently obvious to justify the teleological hypothesis here pro-
posed ; and in the rest of the class the like benefit may result from this arrangement of
the renal veins to a degree corresponding with the necessity for it which may exist.

In the Apteryx the great renal vein (s,Pl.IV.) is not embedded in the substance, but is
continued along the anterior or under-surface of the kidney, receiving the blood from
the lobules of the gland by many oblique but wide openings ; the venous trunks of the
two kidneys anastomose, as in other birds, posteriorly, to form the commencement of
the mesenteric vein (t, Pl. IV.) ; and, anteriorly, after receiving the iliac veins, they unite
to form the vena cava (u), and thus complete the great circulus venosus renalis. The modi-
fications of this part of the venous system were less important than I had been led
to anticipate in a bird whose comparatively limited powers of locomotion must be at-
tended with less partial and excessive action of the respiratory system than in birds of
flight.

The organs of respiration in birds are so eminently characteristic of that class, and
so obviously framed with especial reference to the faculty of aerial progression, that
in the Apteryx - a bird of nocturnal and burrowing habits, and of which the wings
are reduced to the most rudimental condition, - the examination of the associated
modifications of the respiratory system promised to be replete with peculiar in-
terest. It was, in fact, the first point to which I directed my attention, and having
made a preparatory inflation of the pulmonary organs by the trachea, I proceeded to
open the abdomen, and displaced the viscera with great care ; but, as has been already
stated, there was not any trace of the extension of air-cells in the interspaces of the ab-
dominal viscera ; and the whole of them having been removed, I was not less gratified
than surprised to find a complete and well-developed diaphragm separating the abdo-
minal from the respiratory cavity. This septum did not present any large openings
corresponding to those by which the air is continued into the abdomen in the other Stru-
thious birds, but was here perforated only for the transmission of the oesophagus and
large blood-vessels.

The diaphragm of the Apterys differs from that which characterizes the class Mammalia
in the following points ; first, in the greater relative extent of the anterior or post-
sternal interspace ; secondly, in the greater proportion of tendinous or aponeurotic tissue
which enters into its composition ; thirdly, in being perforated by three different large
arteries, and not by the vena cava or splanchnic nerves ; and lastly, in the different rela-
tive positions of the oesophageal and aortic openings. the plane of the diaphragm is
more horizontal, or rather more parallel with the axis of the trunk, than in the Mam-
malia generally ; but some of the aquatic species, as the Dugong, present a position of
the diaphragm almost similar to that of the Apteryx.

The origins of the vertebral or lumbar portion of the diaphragm are by two well-
developed crura (Pl. VI. a, fig. 1.), which are attached to slight prominences on the