OF THE SOUTHERN APTERYX. 275 
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. 
Suppose the Eagle to be glutted with his quarry and reduced to a state of stupor ; the 
animal functions are now at rest, but the organic powers concerned in the assimilation 
of the food are in full play, and the portal or hepatic circulation is as active as was the 
pulmonary a short time before. 
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 respectively emptied, or in other words, 
according to the 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 kidneys is collected from all parts of the gland into a venous reservoir or trunk 
extending longitudinally through the substance of the kidney, and more or less sub- 
divided at the anterior or thick part of the gland 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, in fact, 
the commencement of the mesenteric veins, receiving the blood from the rectum and 
ceca. 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 reasonably sup- 
pose that a greater quantity of the renal blood will flow by the anterior outlets into that 
great channel. 
202 
