the body, or chest, which they always fill. In general, then, he who has a large chest has large lungs.

The size of the chest, and of its contents, depends on a great variety of causes. Among these are hereditary transmission, education, employment, and habit. Men who speak much, use their arms much, or labor much in the open air, generally have large lungs; while females, and those of our own sex who lead a sedentary, studious, or merely contemplative life, have their lungs less fully developed.

once.

I have said that we ordinarily inhale about a pint of air at The lungs are seldom quite full, and still less frequently are they quite exhausted. By an effort we can inhale several pints. It is seldom, however, that, in a quiescent state, the lungs hold a gallon.

The lungs, thus filling the cavity of the chest, rest on an arched or umbrella-shaped bridge, which crosses the body

obliquely, and separates the cavity of the chest from the cavity of the abdomen. This arched partition or bridge is called the diaphragm. It is, of course, convex upwards, and is, like the walls of the stomach, at once membranous and muscular.

When we are about to inhale air, the muscular fibres in this membrane contract, or shrink, in such a manner as to bring the diaphragm more nearly to a level or plane than it was

before, and thus to enlarge the cavity of the chest, creating, in the first instant, a vacuum. But a vacuum, as it is well known, nature every where abhors; and as soon as there is a tendency in this direction, the air rushes in through the mouth and nostrils, and then through the windpipe, which lies in the fore part of the throat, and fills it. Literally the wind blows into the lungs.

Whether the word windpipe had its origin in this fact, I am not informed. A chimney flue or a stove pipe is a windpipe on a somewhat similar principle, only on a larger scale. There is this difference, however: the current of air is set a-going in one case, in consequence of rarefying the air by heat, and in the other by forcibly forming a vacuum.

In forming this vacuum, however, the diaphragm should be assisted by a set of short muscles, called intercostals, lying between the ribs, and so applied that, when they all contract,

and they all do contract, when any one of them does, they enlarge the whole cavity. They increase both its depth and its circumference. This, I say, should be the case. But owing to bad habits, induced by early education and in various

ways, they seldom contract much; so that what is done at all is chiefly done by the diaphragm. We shall see, more particularly, why this is so, hereafter.

The air, having entered the lungs through the windpipe, is carried through the bronchial tubes. These last are simply branches of the windpipe. They begin at the top of the

chest. The lungs are in two great divisions, one for each side of the chest. The bronchial tubes divide and subdivide into branches almost innumerable.

These hollow branches of the windpipe- the bronchial tubes at length terminate in clusters of air bladders, in size exceedingly small, but in number beyond computation.

They have a slight resemblance, in shape, to clusters of grapes, but are, of course, much smaller. Some of them are not the hundredth part of an inch in diameter.

These air bladders, we have seen, are never wholly empty, nor are they often so full that no more air can be admitted. They should be filled at all times, if possible; otherwise the lungs, sooner or later, become diseased.

II. OFFICES OR USES OF THE LUNGS.

The lungs have many offices; some of more, others of less importance. There are, however, but three or four which anatomists and physiologists usually mention.

1. One of the most important offices of the lungs is to receive the chyle, and form it into blood. We might be tempted to regard this as a part of the digestive process. The truth is, it properly belongs both to digestion and respiration.

How is the chyle formed into blood? We are accustomed to say in general, without well understanding the nature of the process, that it is by the union of oxygen in the air cells of the lungs with that admixture of chyle and venous blood which is always found there, in such a manner as to impart vital energy to the chyle, and fit it to aid in building up or renewing the system.