01 apteryx australis - University of Texas Libraries

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by two or three short chordæ tendinea to the angle between the free and fixed parietes
of the ventricle. We perceive inthis mode of connection an approach in the present
bird to the mammalian type of structure analogous to that which the Ornithorhynchus,
among Mammalia, offers, in the structure of the same part, to the class of birds ; for
the right auriculo-ventricular valve in the Ornithorhynchus is partly fleshy and partly
membranous. The dilatable or free parietes of the right ventricle were about 1/20th of
an inch in thickness, those of the left were 1/6th of an inch thick.
There was nothing worthy of note in the left auricle (fig. 2 and 3 h,) or in the valves
interposed between itand the left ventricle : the two membranous flaps presented the
usual inequality of size characteristic of the mitral valve inbirds.
The aorta divides as usual, immediately after its origin, into the ascending and descending
aorta :the ascending aorta as quickly branches into the arteriæ innominate
(d, fig. 2.), which diverge as they ascend and give off the subclavians in the form of
very small branches ; they are then continued, very little diminished in size, as the
carotids ;each carotid divides or gives offa large vertebral artery before passing out of
the thorax ;they then mount upon the neck, converge and enter the inferior vascular
canal of the thirteenth cervical vertebra, and are continued in the interspace of the
hæmapophyses to the fourth cervical vertebra : here they emerge from the subvertebral
canal, and passing through the interspace of the recti capitis antici, they again diverge,
and when opposite the angle of the jaw, give off occipital, internal carotid, large palatine,
and other branches, as in the Emeu. The principal difference observed in the
Apteryx was the equality of size in the carotids :in the EmeuIfound the right carotid
larger than the left.
The descending or third primary division of the aorta (k, fig. 2.) presents in the
Apteryx, as in the Emeu and other Struthionidæ, more of the character of the continuation
of the main-trunk than in the rest of the class, inconsequence of its greater
size and thicker tunics, which relate of course to the diminished supply of blood
transmitted to the rudimental anterior extremities ; and the increased quantity required
to be sent to the powerfully developed legs. The aorta arches over the right
bronchus as usual, and is continued down the thorax to the interspace of the crura of
the diaphragm, through which it passes into the abdomen in a manner remarkably analogous
to that which characterizes the course of the aorta in the Mammalia (Pl. VI.
n, fig. 1). The Apteryx, in fact, seems to be the only bird in which the limits of
thoracic and abdominal aorta can be accurately defined. But, in thus establishing this
distinction, we observe a remarkable difference from the mammalian arterial system, in
the fact, that some large and important branches, which in the latter are given offfrom
the abdominal aorta, arise in the present bird above the diaphragm, through which
they pass by distinct and proper apertures to the abdominal viscera which they are destined
to supply. These branches are the cæliac axis (Pl. VI.l,fig. 1.), and the great
or superior mesenteric artery (m, fig. 1.). Besides these branches, the thoracic aorta
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