ANATOMY AND PHYSIOLOGY. 149 
respects. As it goes it gives life; it brings at the 
same time the air and the materials for repair; and it 
gathers.up the various effete matters, each to be 
thrown off at the gate open only to that particular 
kind of impurity. Their blood is their life, and death 
soon follows the interruption of its flow. 
But in insects all this is altered. The blood is made 
from the food at once, by simple secretion through the 
coats of the stomach and bowels into the irregular 
venous sinuses in which these organs lie. Here, with- 
out any imtermediate elaboration, the fluid chyle, or 
blood, or both, as we may choose to call it, is received; 
and here, side by side, in the fluid, lie the air tubes 
which are to bring oxygen to it and the gland tubes 
which are to extract secretions fromit. These several 
processes, which are, as a rule, kept so distinct in the 
higher animals, are here transacted, not merely in close 
proximity, but actually, as in the placental circulation, 
in the same vessels. 
Again, the circulation in insects is carried on at a 
low pressure, and with none of that force which makes 
the blood spurt out in jets when an artery is divided. 
The reason is plain. Within the hard case there is 
little resistance to be overcome from surrounding 
pressure. Then again, as there are the means of 
oxygenation ready everywhere, there is no need for 
the blood to be hurried round the circle to the pomt 
where it may have its exhausted vitality renewed. 
So no more force is applied than will just keep it 
moving. 
And lastly, with regard to the nature and the 
functions of the blood itself.* It is a transparent 
* ¢ Cyclopedia of Anatomy and Physiology,’ Vol. II, p. 979. 
