We take air into the lungs to pass thence into the blood, and thus be carried to all the cells of the body to enable them to live and breathe.
The Heart.—The heart is at the lower part of the chest, between the two lungs. It is a fleshy or muscular organ, about the size of the fist—flat above, and pointed below like a sugar-loaf. It lies in a slanting direction behind the breastbone—the broad part, or the base, of the heart being upwards and partly to the right of the breast-bone; the point, or apex of the heart, being downwards and to the left, where it can often be seen beating against the chestwall.
The heart is hollow, and acts like a pump, forcing the blood all over the body through the great vessel that leaves the heart at the upper part. The heart, like the lungs, is enclosed in a double layer of folded bag, called the pericardium, because it is round the heart.
The gullet runs right down the back of the thorax, and passes out through the diaphragm, which forms the floor, into the abdomen.
The abdomen forms the lower half of the trunk, and is often called the stomach. It is full of organs belonging to the digestive system and secretory system, by which the fuel or food is rendered fit for use in the blood and the body.
The walls of the abdomen are not protected by ribs like the thorax, but are all formed of flesh or muscle. The principal organs they contain are the stomach, the liver, the pancreas, or sweetbread, the spleen or milt, the kidneys, the intestines, and the bladder.
The Human Brain.—The head and spine contain the principal nervous systems of the body and four organs of special sense—sight, hearing, smelling, and tasting.
The brain, which fills the head, consists of two parts: the Cerebrum, or greater brain, and the Cerebellum, or lesser brain, placed behind and below the larger one. From this brain, nerves run to every muscle of the body, enabling them to move the limbs and body as the mind directs; and another set of nerves run from every part of the body and skin to the brain, enabling the mind to know and feel all that goes on.
The brain is connected with the spinal cord by a flat band of brain matter, that lies on the inside of the occipital bone, called the Medulla Oblongata, or the Oblong Marrow. The spinal cord runs through a large hole in the occipital bone and right down the open tube formed by the spinal vertebræ, to the bottom of the backbone, and, all along its course, nerves leave it and enter it, as in the brain.
The organ of sight consists of the two eyes, which receive every image that we see, and transmit it to the brain. The organ of hearing consists of the two ears, by which we receive all the waves of sound that we hear, and transmit them to the brain. The organ of smell is in the upper part of the nose; the organ of taste at the hinder part of the tongue.