_A._ A natural transition would be from the head to the heart, and, in connection, the circulation of the blood.

_I._ Yes. I will give you an abstract of the ideas I gained in the study of Hayward's Physiology, and the reading of Dr. Paley's Theology. The heart, arteries, and veins are the agents of circulation. The heart is irregular and conical in its shape; and it is hollow and double.

_A._ There is no channel of communication between these parts, is there?

_I._ None; but each side has its separate office to perform. By the right, circulation is carried on in the lungs; and by the left through the rest of the body. I will mark a few pa.s.sages in Paley, for you to read to us, Ann. They will do better than any descriptions of mine.

_A._ I thank you, Isabel, for giving me an opportunity to lend you temporary relief.--"The disposition of the blood-vessels, as far as regards the supply of the body, is like that of the water-pipes in a city, viz. large and main trunks branching off by smaller pipes (and these again by still narrower tubes) in every direction and towards every part in which the fluid which they convey can be wanted. So far, the water-pipes which serve a town may represent the vessels which carry the blood from the heart. But there is another thing necessary to the blood, which is not wanted for the water; and that is, the carrying of it back again to its source. For this office, a reversed system of vessels is prepared, which, uniting at their extremities with the extremities of the first system, collects the divided and subdivided streamlets, first by capillary ramifications into larger branches, secondly by these branches into trunks; and thus returns the blood (almost exactly inverting the order in which it went out) to the fountain whence its motion proceeded. The body, therefore, contains two systems of blood-vessels, arteries and veins.

"The next thing to be considered is the engine which works this machinery, viz., the _heart_. There is provided in the central part of the body a hollow muscle invested with spiral fibres, running in both directions, the layers intersecting one another. By the contraction of these fibres, the sides of the muscular cavity are necessarily squeezed together, so as to force out from them any fluid which they may at that time contain: by the relaxation of the same fibres, the cavities are in their turn dilated, and, of course, prepared to admit every fluid which may be poured into them. Into these cavities are inserted the great trunks both of the arteries which carry out the blood, and of the veins which bring it back. As soon as the blood is received by the heart from the veins of the body, and _before_ that is sent out again into its arteries, it is carried, by the force of the contraction of the heart, and by means of a separate and supplementary artery, to the lungs, and made to enter the vessels of the lungs, from which, after it has undergone the action, whatever it may be, of that viscus, it is brought back, by a large vein, once more to the heart, in order, when thus concocted and prepared, to be thence distributed anew into the system.

This a.s.signs to the heart a double office. The pulmonary circulation is a system within a system; and one action of the heart is the origin of both. For this complicated function four cavities become necessary, and four are accordingly provided; two called ventricles, which _send out_ the blood, viz., one into the lungs in the first instance, the other into the ma.s.s, after it has returned from the lungs; two others also, called auricles, which receive the blood from the veins, viz. one as it comes from the body; the other, as the same blood comes a second time after its circulation through the lungs."

_I._ That must answer our purpose, dear Ann. Of the change which takes place in the blood, and of the renewal of our physical system, which is effected by circulation, I shall say nothing. We will pa.s.s to respiration.

_E._ Whose popular name is breathing?

_I._ Yes. The act of inhaling air, is called inspiration; that of sending it out, expiration. Its organs are the lungs and windpipe. The apparatus employed in the mechanism of breathing is very complex. The windpipe extends from the mouth to the lungs.

_A._ How is it that air enters it so freely, while food and drink are excluded?

_I._ By a most ingenious contrivance. The opening to the pipe is called glottis. This is closed, when necessary, by a little valve, or lid, called the epiglottis (_epi_ means _upon_.)

_E._ And this faithful sentinel is none other than that perpendicular little body which we can see in our throats, and which we have _dubbed_ palate.

_I._ You are right, Ellinora. Over this, food and drink pa.s.s on their way to the road to the stomach, the gullet. The pressure of solids or liquids tends to depress this lid on the glottis; and its muscular action in deglut.i.tion, or swallowing, tends to the same effect. As soon as the pressure is removed, the lid springs to its erect position, and the air pa.s.ses freely. Larynx and trachea are other names for the windpipe, and pharynx is another for the gullet. The larynx divides into two branches at the lungs, and goes to each side. Hence, by subdivisions, it pa.s.ses off in numerous smaller branches, to different parts of the lungs, and terminates in air-cells. The lungs, known in animals by the name of lights, consist of three parts, or lobes, one on the right side, and two on the left.

_Alice._ The lights of inferior animals are very light and porous--do our lungs resemble them in this?

_I._ Yes; they are full of air-tubes and air-cells. These, with the blood vessels and the membrane which connects (and this is cellular, that is, composed of cells,) form the lungs. The process of respiration involves chemical, mechanical, and vital or physiological principles. Of the mechanism I shall say but little more. You already know that the lungs occupy the chest. Of this, the breast bone forms the front, the spine, the back wall. Attached to this bone are twelve ribs on each side. These are joined by muscles which are supposed to a.s.sist in elevating them in breathing, thus enlarging the cavity of the chest. The lower part.i.tion is formed by a muscle of great power, called the diaphragm, and by the action of this organ alone common inspiration can be performed. Hayward says, "The contraction of this muscle necessarily depresses its centre, which was before elevated towards the lungs. The instant this takes place, the air rushes into the lungs through the windpipe, and thus prevents a vacuum, which would otherwise be produced between the chest and lungs." Expiration is the reverse of this. The chemistry of respiration regards the change produced in the blood by respiration. To this change I have before alluded.

_Ann._ When we consider the offices of the heart and lungs, their importance in vital economy, how dangerous appears the custom of pressing them so closely between the ribs by tight lacing?

_I._ Yes; fearful and fatal beyond calculation! And one great advantage in a general knowledge of our physical system, is the tendency this knowledge must have to correct this habit.

_A._ To me there is not the weakest motive for tight lacing. Everything but pride _must_ revolt at the habit; and there is something positively disgusting and shocking in the wasp-like form, labored breathing, purple lips and hands of the tight lacer.

_E._ They indicate such a pitiful servitude to fashion, such an utter disregard of comfort, when it comes in collision with false notions of elegance! Well for our s.e.x, as we could not be induced to act from a worthier motive, popular opinion is setting in strongly against this practice. Many of our authors and public lecturers are bringing strong arms and benevolent hearts to the work.

_A._ Yes; but to be perfectly consistent, should not the fashions of the "Lady's Book," the "Ladies' Companion," and of "Graham's Magazine," be more in keeping with the general sentiment? Their contributors furnish essays, deprecating the evils of tight lacing, and tales ill.u.s.trative of its evil effects, yet the figures of the plates of fashions are uniformly most unnaturally slender. And these are offered for national standards!

_E._ "And, more's the pity," followed as such.

_I._ I think the improvements you mention would only cause a temporary suspension of the evil. They might indeed make it the _fashion_ to wear natural waists; but like all other fashions, it must unavoidably give way to new modes. They might lop off a few of the branches; but science, a knowledge of physiology alone, is capable of laying the axe at the root of the tree.--What is digestion, Ellinora?

_E._ It is the dissolving, pulverizing, or some other _ing_, of our food, isn't it?

_I._ Hayward says that "it is an important part of that process by which aliment taken into the body is made to nourish it." He divides the digestive apparatus into "the mouth and its appendages, the stomach and the intestines." The teeth, tongue, jaws, and saliva, perform their respective offices in mastication. Then the food pa.s.ses over the epiglottis, you recollect, down the gullet to the stomach. The saliva is an important agent in digestion. It is secreted in glands, which pour it into the mouth by a tube about the size of a wheat straw.

_Alice._ I heard our physician say that food should be so thoroughly masticated before deglut.i.tion (you see I have caught your technicals, Isabel,) that every particle would be moistened with the saliva. Then digestion would be easy and perfect. He says that dyspepsia is often incurred and perpetuated by eating too rapidly.

_I._ Doubtless this is the case. As soon as the food reaches the stomach, the work of digestion commences; and the food is converted to a ma.s.s, neither fluid or solid, called chyme. With regard to this process, there have been many speculative theories. It has been imputed to animal heat, to putrefaction, to a mechanical operation (something like that carried on in the gizzard of a fowl,) to fermentation, and maceration.

It is now a generally adopted theory, that the food is _dissolved_ by the gastric juices.

_Ann._ If these juices are such powerful solvents, why do they not act on the stomach, when they are no longer supplied with _subjects_ in the shape of food?

_I._ According to many authorities, they do. Comstock says that "hunger is produced by the action of the gastric juices on the stomach." This theory does not prevail, however; for it has been proved by experiment, that these juices do not act on anything that has life.

_Alice._ How long does it take the food to digest?

_I._ Food of a proper kind will digest in a healthy stomach, in four or five hours. It then pa.s.ses to the intestines.

_Ann._ But why does it never leave the stomach until thoroughly digested?

_I._ At the orifice of the stomach, there is a sort of a valve, called pylorus, or door-keeper. Some have supposed that this valve has the power of ascertaining when the food is sufficiently digested, and so allows chyme to pa.s.s, while it contracts at the touch of undigested substances.

_A._ How wonderful!

_I._ And "how pa.s.sing wonder He who made us such!"

_Alice._ No wonder that a poet said--

"Strange that a harp of thousand strings Should keep in tune so long!"

_Ann._ And no wonder that the Christian bends in lowly adoration and love before _such_ a Creator, and _such_ a Preserver?

_E._ Now, dear Isabel, will you tell us something more?

_I._ Indeed, Ellinora, I have already gone much farther than I intended when I commenced. But I knew not where to stop. Even now, you have but just _commenced_ the study of _yourselves_. Let me urge you to read in your leisure hours, and reflect in your working ones, until you understand physiology, as well as you now do geography.

D.

[Ill.u.s.tration: Decoration]