I have said already that the chest and abdomen are popular divisions of the trunk; but there is a very good reason for this - namely that in us, and in all animals of the class to which we belong, the mammalia - so called because they nourish their own young - the chest and the abdomen are divided from one another by a partition, which goes by the name of the diaphragm.

I have mentioned the chief separate contents of the chest and abdomen. Besides these there is a double chain of nerves with knots (which we call ganglia) upon them, running down through the thorax or chest, and through the abdomen, behind all these organs. This double chain of nerves goes by the name of the sympathetic nerves. But if you took a human being, and examined him right through, from front to back, in the chest or abdomen, you would also find a chain of bones running from the head downwards behind the cavities of the chest and abdomen; and behind that chain of bones, a tube running right down, and inside that tube, a white cord which we call the spinal cord; and if you looked, farther, you would see that the spinal cord is continuous above with part of the brain, through a hole in the walls of the cranium.

The Extremities or limbs have no such cavities containing special organs as are in the head and trunk; they are solid throughout, except that they contain certain tubes. They, in fact, are made up of the same kind of structures as the walls of the body generally these structures we will now consider.

In the first place, the whole exterior of the body is covered by what we call the skin. In the skin there are two important layers: there is the skin proper called the dermis, which is soft, moist, very sensitive and supplied with a great deal of blood. It bleeds when it is cut. There is also a covering to the skin proper which we call the epidermis, because it is upon the dermis, or outside of the skin proper. This is drier, not sensitive, not supplied with blood, and consists of horny scales which are continually falling off. All the surface of the body is covered with it. Wherever there is an opening from the interior of the body to the external air, the skin, as we understand it, ceases, and another substance, or rather something that we call by another name, takes its place. All the cavities of the body that communicate with the external air are lined by a kind of internal skin, which we call the mucous membrane. This internal skin resembles the external in that it has two important layers. It has a deep layer, which is soft, sensitive, well supplied with blood, and a superficial layer, which is moist, but otherwise not unlike the superficial layer of the skin itself, as it is insensitive and not supplied with blood; thus, at all the openings between the internal organs that communicate with the external air and the surface, this mucous membrane, which lines these internal organs, joins the skin, so that we see that the organs already described as in the body, and many that we have not mentioned are, as it were, contained between these two skins.

The superficial layer of the mucous membrane is called the epithelium; so that in the skin we have the dermis or true skin, and the epidermis, which is some-times called the scarf skin; and in the mucous membrane a deep layer, and a superficial layer called the epithelium. Underneath the skin of the body we find, in the first place, a more or less thick layer of fat, which is thicker in certain parts of the body than in others, and beneath that a number of structures that we call muscles or flesh.

We find this whether we take the extremities, or the walls of the body anywhere, and we find besides this a number of tubes containing blood, called blood-vessels, in all parts of the body below the epidermis or below the epithelium, and other tubes not containing blood, but a nearly colourless fluid, which, from its resemblance to water, is called lymph. These tubes go by the name of lymphatic ducts. Besides them we find white cords in almost all parts of the body, more or less directly connected with the brain or the spinal cord, which we call nerves. These are the structures that we find in the limbs, and also wherever we examine the walls of the body.

Such is a very brief outline of the more important organs of the body, and the way in which they are put together.

We will now pass on to consider parts of the body more particularly, and will begin with the bones. In the young human body there are more than two hundred separate bones, but some of these grow together in the adult, so that several form one bone.