17 
by two or three short chorde tendinee to the angle between the free and fixed parietes 
of the ventricle. We perceive in this 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 Ornithurhynchus is partly fleshy and partly 
membranous. The dilatable or free parietes of the right ventricle were about 4th of 
an inch in thickness, those of the left were 4th 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 it and the left ventricle: the two membranous flaps presented the 
usual inequality of size characteristic of the mitral valve in birds. 
The aorta divides as usual, immediately after its origin, into the ascending and de- 
scending aorte: the ascending aorta as quickly branches into the arteri@ 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 dimimshed in size, as the 
carotids ; each carotid divides or gives off a 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 
hamapophyses to the fourth cervical vertebra: here they emerge from the subvertebral 
canal, and passing through the interspace of the rect: capitis antici, they again diverge, 
and when opposite the angle of the jaw, give off occipital, internal carotid, large pala- 
tine, and other branches, as in the Emeu. The principal difference observed in the 
Apteryx was the equality of size in the carotids: in the Emeu I found 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 Struthionide, more of the character of the conti- 
nuation of the main-trunk than in the rest of the class, in consequence 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 re- 
quired 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 ana- 
logous to that which characterizes the course of the aorta in the Mammalia (Pl. V1. 
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 off from 
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 des- 
tined to supply. These branches are the celiac avis (P|. VI. /, fig. 1.), and the great 
or superior mesenteric artery (m, fig. 1.). Besides these branches, the thoracic aorta 
D 
