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renal veins which regulates the quantity of blood transmitted to the lungs or to the liver
respectively, as in other birds. This disposition has been erroneously supposed to in-
dicate that the urine was secreted from the venous blood in birds, as in reptiles and
fishes ; but the end attained by the venous anastomoses in question bears a much closer
relation to the peculiar necessities and habit of life of the bird, and so far as I know,
has not hitherto been explained. There is no class of animals in which there may be,
at any two brief and consecutive periods of existence, a greater difference in the degree
of energy and rapidity with which the respiratory functions are performed, than in birds.
When the bird of prey, for example, stimulated by a hungry and an empty stomach,
soars aloft and sweeps the air in quest of food, the muscular energies are then
strained to the utmost, the heart beats with the most forcible and rapid contractions to
propel the current of blood along the systemic arteries, and the pulmonary vessels
require the greatest possible supply of blood to serve the heart with the due quantity
of arterialized fluid : the digestive system, on the other hand, is in a state of repose,
and we may conceive the portal circulation to be at its lowest ebb.

But when the Eagle is glutted with his quarry and reduced to a state of stupor, there
is a reverse condition of the two great systems which propel the venous blood from trunks
to branches : the animal functions are now at rest, while the organic powers concerned
in the assimilation of the food are in full play, and the portal or hepatic circulation now
demands as great a supply as did that of the lungs under the previous condition.

The venous system of the kidneys is so arranged in birds that it can be distributed
either to the portal system by the mesenteric vein, or to the pulmonary system by the
vena cava and right side of the heart, according to the degree of rapidity with which
the pulmonary or portal systems of veins are repectively emptied, or in other words,
according to te activity with which the circulation in each of these systems may be
going on at two different periods. The arrangement is as follows : the venous blood of
the kidney is collected into a venous reservoir or trunk extending longitudinally through
the substance of the gland, and more or less subdivided at its anterior or thick part
in most birds ; here it communicates by one or more large anastomoses with the iliac
vein, which, after a short course, unites with its fellow to form the trunk of the vena cava ;
at the posterior or lower end of the kidney the renal vein emerges, and after receiving
some small veins from the cloaca, joins the vein from the opposite kidney, and the
common trunk, thus formed, then bends forwards, enters the folds of the mesentery
of the rectum, and becomes the commencement of the mesenteric veins, receiving the
blood from the rectum and coeca. Thus, when the circulation of the portal system is
unusually active, the current of the venous blood of the kidneys will naturally tend
towards the lower outlet into the mesenteric vein; but when, on the other hand, those
causes are in operation which accelerate the current of venous blood through the vena
cava, we may resonably suppose that a greater quantity of the renal blood will flow by
the anterior outlets into that great channel.

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