opens into a chamber with cartilaginous walls the larynx; and leading from the larynx downwards along the front part of the throat, where it may be very readily felt, is the trachea, or windpipe (Fig. 19, Tr.).

FIG. 19.-BACK VIEW OF THE NECK AND THORAX OF A HUMAN SUBJECT FROM WHICH THE VERTEBRAL COLUMN AND WHOLE POSTERIOR WALL OF THE CHEST ARE SUPPOSED TO BE REMOVED.

M mouth; Gl. glottis; Tr. trachea; L.L. left lung; R.L. right lung; Br. bronchus; P.A. pulmonary artery; P. V. pulmonary veins; Ao. aorta; D. diaphragm; H. heart; V.C.I. vena cava inferior.

If the trachea be handled through the skin, it will be found to be firm and resisting. Its walls are, in fact, strengthened by a series of cartilaginous hoops, which hoops are incomplete behind, their ends being united only by muscle and membrane, where the trachea comes into contact with the gullet, or esophagus. The trachea passes into the thorax, and there divides into two branches, a right and a left, which are termed the bronchi (Fig. 19, Br.). Each bronchus enters the lung of its own side

and then breaks up into a great number of smaller branches, which are called the bronchial tubes. As these diminish in size, the cartilages, which are continued all through the bronchi and their large ramifications, become smaller and eventually disappear, so that the walls of the smallest bronchial tubes are entirely muscular or membranous. Thus while the trachea and bronchi are kept permanently open and pervious to air by their cartilages, the smaller bronchial tubes may be almost closed by the contraction of their muscular walls.

The finer bronchial tubes end at length in elongated dilatations, about 6th of an inch in diameter on the average (Fig. 20, A). Each of these dilatations is beset with, or perhaps rather is made up of, little sacs, which open irregularly into the cavity of the dilatation. These sacs are the air-cells. The very thin walls (Fig. 20, B) which separate these air-cells are supported by much delicate and highly elastic tissue, and carry the wide and close-set capillaries into which the ultimate ramifications of the pulmonary artery pour its blood (Fig. 20, D). Thus, the blood contained in these capillaries is exposed on both sides to the air-being separated from the air-cell on either hand only by the very delicate pellicle which forms the wall of the capillary, and the lining of the air-sac.

9. Hence no conditions can be more favourable to a ready exchange between the gaseous contents of the blood and those of the air in the air-cells, than the arrangements which obtain in the pulmonary capillaries; and, thus far, the structure of the lung fully enables us to understand how it is that the large quantity of blood poured through the pulmonary circulation becomes exposed in very thin streams, over a large surface, to the air. But the only result of this arrangement would be, that the pulmonary air would very speedily lose all its oxygen, and become completely saturated with carbonic acid, if special provision were not made for its being incessantly renewed.

10. If an adult man, breathing calmly in the sitting position, be watched, the respiratory act will be observed to be repeated thirteen to fifteen times every minute. Each act consists of certain components which succeed one another in a regular rhythmical order. First, the breath is drawn in, or inspired; immediately afterwards

G

it is driven out, or expired; and these successive acts of inspiration and expiration are followed by a brief pause. Thus, just as in the rhythm of the heart the auricular systole, the ventricular systole, and then a pause follow in

A. Two air-cells (6) with the ultimate bronchial tube (a) which opens into them. (Magnified 20 diameters.)

B. Diagrammatic view of an air-cell of A seen in section: a, epithelium; b, partition between two adjacent cells, in the thickness of which the capillaries run; c, fibres of elastic tissue.

C. Portion of injected lung magnified: a, the capillaries spread over the walls of two adjacent air-cells; b, small branches of arteries and veins,

D. Portion still more highly magnified.

regular order; so in the chest, the inspiration, the expiration, and then a pause succeed one another. At each inspiration of an adult well-grown man about thirty cubic inches of air are inspired; and at each expiration the same, or a slightly smaller, volume (allowing for the increase of temperature of the air so expired) is given out of the body.

II. The expired air differs from the air inspired in the following particulars :

(a) Whatever the temperature of the external air is, that expired is nearly as hot as the blood, or has a temperature between 90° and 100°.

(6) However dry the external air may be, that expired is quite, or nearly, saturated with watery vapour.

(c) Though ordinary air contains nearly 2,100 parts of oxygen, and 7,900 of nitrogen, with not more than 3 parts of carbonic acid, in 10,000 parts, expired air contains about 470 parts of carbonic acid, and only between 1,500 and 1,600 parts of oxygen; while the quantity of nitrogen suffers little or no change. Speaking roughly, air which has been breathed once has gained five per cent. of carbonic acid, and lost five per cent. of oxygen.

The expired air contains, in addition, a greater or less quantity of animal matter of a highly decomposable character.

(d) Very close analysis of the expired air shows, firstly, that the quantity of oxygen which disappears is always slightly in excess of the quantity of carbonic acid supplied; and secondly, that the nitrogen is variable-the expired nitrogen being sometimes slightly in excess of, sometimes slightly less than that inspired, and sometimes remaining stationary.

12. From three hundred and fifty to four hundred cubic feet of air are thus passed through the lungs of an adult man taking little or no exercise, in the course of twentyfour hours; and are charged with carbonic acid, and deprived of oxygen, to the extent of nearly five per cent. This amounts to about eighteen cubic feet of the one gas taken in, and of the other given out. Thus, if a man be shut up in a close room, having the form of a cube seven feet in the side, every particle of air in that room will have passed through his lungs in twenty-four hours, and

a fourth of the oxygen it contained will be replaced by . carbonic acid.

The quantity of carbon eliminated in the twenty-four hours is pretty clearly represented by a piece of pure charcoal weighing eight ounces.

The quantity of water given off from the lungs in the twenty-four hours varies very much, but may be taken on the average as rather less than half a pint, or about nine ounces. It may fall below this amount, or increase to double or treble the quantity.

13. The mechanical arrangements by which the respiratory movements, essential to the removal of the great mass of effete matters, and the importation of the large quantity of oxygen indicated, are effected, may be found in (a) the elasticity of the lungs ; (b) the mobility of the sides and bottom of the thoracic cavity in which the lungs are contained.

The thorax may be regarded as a completely shut conical box, with the small end turned upwards, the back of the box being formed by the spinal column, the sides by the ribs, the front by the breast-bone, the bottom by the diaphragm, and the top by the root of the neck (Fig. 19).

The two lungs occupy almost all the cavity of this box which is not taken up by the heart. Each is enclosed in its serous membrane, the pleura, a double bag (very similar to the pericardium, the chief difference being that the outer bag of each pleura is, over the greater part of its extent, quite firmly adherent to the walls of the chest and the diaphragm (see Fig. 9), while the outer bag of the pericardium is for the most part loose), the inner bag closely covering the lung and the outer forming a lining to the cavity of the chest. So long as the walls of the thorax are entire, the cavity of each pleura is practically obliterated, that layer of the pleura which covers the lung being in close contact with that which lines the wall of the chest; but if a small opening be made into the pleura, the lung at once shrinks to a comparatively small size, and thus develops a great cavity between the two layers of the pleura. If a pipe be now fitted into the bronchus, and air blown through it, the lung is very readily distended to its full size; but, on being left to itself, it collapses, the air being driven out again with some force.