The engine by which the blood is forced along its vessels is the _heart_! This is divided into two distinct parts, each of which has two cavities, the upper one called the _auricle_, and the lower one the _ventricle_, which communicate with each other by curious valves. In the adult the whole of the impure blood is poured into the right auricle, that from the lower part of the body by the _inferior vena cava_, and that from the upper part by the _superior vena cava_. From the right auricle it passes into the right ventricle, which pumps it into the lungs, by way of the pulmonary artery; here it is purified by the act of respiration, and then brought, when pure, by the pulmonary veins, into the left auricle, and passes from thence into the left ventricle, which pumps it into the great aorta, and from thence into the smaller arteries all over the body. The two sides of the heart, therefore, do not communicate directly with each other, but there is a strong partition between them. In the fœtus the arterial blood from the mother, when it leaves the umbilical artery, enters first the liver, runs through its vessels, gives off the bile found in it, and then joins the vena cava inferior. By this passage it is taken into the right auricle, along with the impure blood of the vena cava. From the right auricle it passes through a hole in the partition directly into the left auricle, instead of taking the indirect route by the lungs as in the adult. From the left auricle it passes into the left ventricle, and is from thence distributed by the arteries all over the body. This opening in the partition is called the _foramen ovale_! After birth, when the blood begins to pass through the lungs, this passage closes up. By the eighth day it is generally obliterated, often much sooner, though occasionally it has remained open longer without inconvenience. In some cases the foramen ovale does not close at all. The child then has what is called the _blue disease_! The whole body is of a uniform leaden, or blue color, and the whole system is generally languid and sluggish. The blue color is caused by the dark blood of the veins mixing with that of the arteries. These children mostly die early, but some live to be five or six years old, and one I saw twelve, but this is rare. No remedy can be had for this affliction, and I have never known it to cure spontaneously. Some children are so very dark for a few days after birth as to cause great alarm. This is owing to the foramen ovale being very open and closing slowly. No apprehension need be experienced in such cases, as it soon subsides.