584 
THE CIRCULATION OF THE BLOOD. 
coloured halo, and, opening the windows, he found that it arose entirely 
from that thin plate of ice, for none was seen except through it. Dr. 
Kotelnihow having, like Dr. Halley, made very accurate observations 
to determine the number of possible rainbows, considers the coloured 
halo which appears about a candle, as the same thing with one of 
those bows which is folmed near the body of the sun, but which is 
not visible on account of his excessive splendour. 
The Circulation of the Blood. 
So important a subject as the circulation of the blood cannot fail 
of being interesting to our general readers ; we shall therefore lay 
before them a brief account of it, divested of that complication which 
renders it only intelligible to anatomists. 
The heart, by the contraction of which the blood is circulated, 
has arising out of it two great blood-vessels, whose branches extend 
to all parts of the body, accompanying each other throughout ; the 
one is the great artery, the aorta^ and the other the great vein, or 
vena cava. The heart has alw ays two other great vessels arising from 
its other side ; one called the great artery of the lungs, or pulmonary 
arteryy the other the great vein of the lungs, or pulmonary vein. Let 
us therefore keep in view that the heart has four large trunks, com- 
municating with it, and that, at the junction of each with the heart, 
there are placed valves, most beautifully perfect, which act in such a 
manner as to admit the tide of blood through its own proper channel, 
in passing and repassing the heart and lungs, and immediately to fly 
up and prevent its improper return, like flood-gates. Arteries are always 
accompanied by veins closely connected together, the arteries carry- 
ing the blood from the heart — the veins carrying it back to it. An 
artery is elastic, and can contract and dilate — a vein is an inactive 
flaccid tube. An artery has no valve in its whole course to the 
extremities of the body— a vein has valves placed at _very short dis- 
tances. These valves are to support the upper column of blood as it 
ascends from below back to the heart, flying up and acting as a floor 
to that portion of blood which is above it, and between the next valve 
and itself ; thus every motion of our limbs moves the blood in their 
veins, and that motion can be no other than upwards, on account of 
those valves ; while the motion of the blood in the arteries is directly 
from the contraction of the heart, and it has a free current to the 
extreme parts of the body. With this general view in mind, let us 
proceed to describe the circulation. 
The blood is sent out at one gush, or pulsation, throughout the 
whole body, into the most minute branches of the arteries; those 
arteries make a turn, and, losing their elasticity, become veins, which 
grow large in proportion as they go towards the heart, and lie exactly 
in the course of their corresponding arteries. Into these veins the 
blood is therefore forced, after having supplied the various secretions 
of the body. This blood is thus brought hack by the great vein, or 
uewa and at its' junction with the left jugular and subclavian 
vein, it receives by a little tube the white chyle or essence of the food 
brought by that tube from the stomach. The blood is then unfit for 
