nutriment of food, in a large duct called the Thoracic Duct, from the digestive organs to a certain part of the large trunk of a vein near the heart, which trunk it enters, but, in truth, it really enters at the place of juncture of adjoining veins, which go to form this, the largest vein of the body; and the chyle or nutriment is thus commingled with the dark stream of venous blood containing waste matters of the body, and soon with it to enter the right side of the heart, and then the lungs. After exposure to atmospheric air inhaled, it is returned, as you see by the dotted line, in its new character of arterial and nutritious blood, from the lungs to the left side of the heart. Next it is propelled into the largest artery of the body, called the Aorta, and is distributed throughout the entire substance of the body, and to every minute and extreme structure, for nutrition, or change of tissue. The arrows mark the direction in which the arterial and the venous blood flow. -And you see that it is in a circle, in a certain sense, as I said before, but not in a literal sense, for the change of tissue must take place in the substance of the heart and lungs themselves, as well as in all other parts of the animal frame.

You must ever remember that veins are blood-vessels which carry very dark red and impure blood from all parts of the body to the heart and lungs; whilst arteries are blood-vessels which carry bright red, or scarlet coloured, and nutritious, vitalized blood from the lungs and heart to all parts of the body.

You can readily understand that the refuse and waste matters of the body, which do not pass off by the lungs, are removed, according to their nature, by their suitable outlets of the bowels, the kidneys, and the skin.

The change of tissue, nutrition, or the renewal of the various structures of the human body, becomes the chief source of health and disease, according to its right or wrong performance. You must consider, however, that it is not the primary source, but secondary to the function of digestion and assimilation of food into blood; and that this, again, is in secondary relation to nervous power. What constitutes this nervous power is not known; only it seems to be closely associated, if not identical, with electricity.

I must now tell you of the principal structures of the human body, to prepare you for further instruction. These are the muscular, the cellular, the osseous or bony, and the nervous.

The muscles of the body are what is commonly called the flesh; and many simple, untaught, and unthinking people suppose that this flesh is a confused mass which surrounds and adheres to the bones; and they are without any definite idea of its use. You know, that this flesh is a regularly arranged system of separate parts or muscles, which are of various sizes and forms; that each muscle is a bundle of fleshy fibres, and is an organ of motion; and has an origin and an insertion. When the body is divested of its covering or skin, you see these masses or bundles of flesh, called muscles, running in different directions. You see, also, that they are of various shapes: some are narrow and thick, others are broad and thin: some are more round in appearance: some are uniform in their size; and others are large in the middle, and taper towards their extremities: some spread out in the form of a fan: some are long, and others are short. They differ, also, in their course or direction: some run parallel with the bones, whilst others go in an oblique or transverse direction. These various particulars of these bundles of flesh, called muscles, adapt them for their various uses in the motions of the different parts of the body.

Each muscle is surrounded by its own separate sheath of a gauze-like cellular tissue; and if you carefully open this sheath, you will find that the substance of the muscle is composed of smaller bundles of parallel fibres; and that each bundle of fibres is also enclosed in a fine sheath of cellular tissue. There is much interesting anatomy regarding muscles; but I must be brief. I have just to state, that every motion of the human body is produced through the instrumentality of muscles. They constitute its principal bulk, symmetry, and comeliness. Remember, that they are developed in proportion to their exercise. Hence, you may see them displayed in their fulness and beauty in the lower limbs of dancers; in the arms of blacksmiths, quarry-men, and navvies: and you may see them in their fullest proportionate and general development in the trained pugilist.

I here present to you a view of the muscles of the trunk of the body, when the skin has been carefully removed.

The cellular tissue forms a kind of minute net-like framework of the whole body; and it enters so minutely and extensively into every part, that if all the other matters were entirely removed, this cellular tissue would perfectly preserve not only the outline, but the exact dimensions of every organ and part. Every bone partakes largely of this cellular tissue in a spongy or network arrangement; the interstices, or vacant places, being filled with the component matter of bone.

This brings me to the very brief notice I am to take of the structure of bones. Without further particulars, I have just to

inform you that they consist of two distinct ingredients: one is gelatine, which has been already alluded to; and it is an animal substance; the other is the hard earthy matter, called phosphate of lime, which gives the quality of hardness to bones. It is soluble in diluted acids: therefore, if you place one of the long bones of the body, say one of the arm bones, in such a liquid of diluted sulphuric, muriatic, or nitric acid, it will lose its phosphate of lime, and the gelatine only will remain: but the dimensions and shape of the bone will be retained. With the phosphate of lime, it will also lose its brittleness and hardness; so that you may then take the same long bone and tie it in a knot, as is represented in the annexed figure.

Membranous portion of bone; the osseous portion being so completely removed, that the bone is capable of being tied in a knot.

There is a certain diseased state of the human constitution called rickets-technically, mollities ossium, or softening of the bones-in which the limbs may be twisted and bended without harm. I shall bring before you such a case in a future letter.

There is also a diseased state of an opposite nature to this, wherein there is a deficiency of gelatine in the structure of the bones, and they consequently become brittle. If you burn a bone in the fire, it will destroy the gelatine, and leave the phosphate of lime. The bone will remain of the same dimensions or size, but it will have become very brittle and very light in weight, from the loss of its gelatine.

I now tell you of the structure of nerves. They are firm white cords, which are directly or indirectly continued from the substance of the brain, spinal marrow, and the other centres or sources of nervous power. Yet, instances have been met with where the brain, and even the spinal marrow, have been found nearly obliterated in the fœtus, or unborn child, and yet the nerves retained their usual appearance.

The nerves are distributed throughout the body, and extend to every part which possesses that sensibility which gives rise to perception, and to every part having an associated action with other parts.

Nerves differ in form, and are more or less of a flattened or oval form; but this is mainly owing to their place and relation, and not to any peculiarity of function. They vary in firmness and density; but this, like their form, depends on their situation; for where they are protected they are more soft, and where they are exposed they have a harder covering.

The matter of nerves is something peculiar, and you will do well to remember that it is, in health, of an opaque white colour; that it is a soft, pulpy matter, being betwixt fluid and solid. By want of use, this matter is either not secreted in due proportion, or it changes its appearance; for the nerves then acquire a degree of transparency.

The nerves have three coats or coverings, which are thought to be continuous with those of the brain. They are composed of sheaths, or most minute tubes, filled with the proper nervous matter, and that matter is a real secretion, and like the other solids and fluids of the body, it is constantly undergoing renewal. This is a very important point, and ought to be re

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